Add DOWNLOAD_RUBY option to Ruby plugin cmake, defaulting to ON

If DOWNLOAD_RUBY is set to OFF, the build will not attempt to download
a prebuilt libruby.so and not fire a warning that one is not present.
This may be desired if one wants to link dfhack against a newer system
libruby.so, now that dfhack supports ruby 2.x and also supports linking
against system libruby (at least, on Linux), as per the following commit:

df9b5bca73

This also allows the dfhack build to proceed without network access
(once all submodules have been fetched), which at least some Linux
distributions, like Fedora, require.

By default DOWNLOAD_RUBY is set to ON, so this should not change
the behavior of default builds.

I tested that when DOWNLOAD_RUBY is set to OFF, dfhack still launches
and that the Ruby plugin still functions, e.g. by running
```rb_eval puts('Hello world.')``` in the dfhack console.

plugins/ruby/CMakeLists.txt

@@ -1,52 +1,59 @@
-IF (APPLE)
-    SET(RUBYLIB ${CMAKE_CURRENT_SOURCE_DIR}/osx${DFHACK_BUILD_ARCH}/libruby.dylib)
-    SET(RUBYLIB_INSTALL_NAME "libruby.dylib")
-    IF(${DFHACK_BUILD_ARCH} STREQUAL 64)
-        # MESSAGE("No ruby lib for 64-bit OS X yet")
-    ELSE()
-        DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/osx32-libruby187.dylib.gz"
-            "gz"
-            ${RUBYLIB}.gz
-            "e9bc4263557e652121b055a46abb4f97"
-            ${RUBYLIB}
-            "3ee5356759f764a440be5b5b44649826")
-    ENDIF()
-ELSEIF(UNIX)
-    SET(RUBYLIB ${CMAKE_CURRENT_SOURCE_DIR}/linux${DFHACK_BUILD_ARCH}/libruby.so)
-    SET(RUBYLIB_INSTALL_NAME "libruby.so")
-    IF(${DFHACK_BUILD_ARCH} STREQUAL 64)
-        DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/linux64-libruby187.so.gz"
-            "gz"
-            ${RUBYLIB}.gz
-            "8eb757bb9ada08608914d8ca8906c427"
-            ${RUBYLIB}
-            "e8c36a06f031cfbf02def28169bb5f1f")
-    ELSE()
-        DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/linux32-libruby187.so.gz"
-            "gz"
-            ${RUBYLIB}.gz
-            "2d06f5069ff07ea934ecd40db55a4ac5"
-            ${RUBYLIB}
-            "b00d8d7086cb39f6fde793f9d89cb2d7")
-    ENDIF()
-ELSE()
-    SET(RUBYLIB ${CMAKE_CURRENT_SOURCE_DIR}/win${DFHACK_BUILD_ARCH}/libruby.dll)
-    SET(RUBYLIB_INSTALL_NAME "libruby.dll")
-    IF(${DFHACK_BUILD_ARCH} STREQUAL 64)
-        DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/win64-libruby200.dll.gz"
-            "gz"
-            ${RUBYLIB}.gz
-            "81db54a8b8b3090c94c6ae2147d30b8f"
-            ${RUBYLIB}
-            "8a8564418aebddef3dfee1e96690e713")
+# Allow build system to turn off downloading of libruby.so.
+OPTION(DOWNLOAD_RUBY "Download prebuilt libruby.so for ruby plugin." ON)
+
+IF (DOWNLOAD_RUBY)
+
+    IF (APPLE)
+        SET(RUBYLIB ${CMAKE_CURRENT_SOURCE_DIR}/osx${DFHACK_BUILD_ARCH}/libruby.dylib)
+        SET(RUBYLIB_INSTALL_NAME "libruby.dylib")
+        IF(${DFHACK_BUILD_ARCH} STREQUAL 64)
+            # MESSAGE("No ruby lib for 64-bit OS X yet")
+        ELSE()
+            DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/osx32-libruby187.dylib.gz"
+                "gz"
+                ${RUBYLIB}.gz
+                "e9bc4263557e652121b055a46abb4f97"
+                ${RUBYLIB}
+                "3ee5356759f764a440be5b5b44649826")
+        ENDIF()
+    ELSEIF(UNIX)
+        SET(RUBYLIB ${CMAKE_CURRENT_SOURCE_DIR}/linux${DFHACK_BUILD_ARCH}/libruby.so)
+        SET(RUBYLIB_INSTALL_NAME "libruby.so")
+        IF(${DFHACK_BUILD_ARCH} STREQUAL 64)
+            DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/linux64-libruby187.so.gz"
+                "gz"
+                ${RUBYLIB}.gz
+                "8eb757bb9ada08608914d8ca8906c427"
+                ${RUBYLIB}
+                "e8c36a06f031cfbf02def28169bb5f1f")
+        ELSE()
+            DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/linux32-libruby187.so.gz"
+                "gz"
+                ${RUBYLIB}.gz
+                "2d06f5069ff07ea934ecd40db55a4ac5"
+                ${RUBYLIB}
+                "b00d8d7086cb39f6fde793f9d89cb2d7")
+        ENDIF()
     ELSE()
-        DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/win32-libruby187.dll.gz"
-            "gz"
-            ${RUBYLIB}.gz
-            "ffc0f1b5b33748e2a36128e90c97f6b2"
-            ${RUBYLIB}
-            "482c1c418f4ee1a5f04203eee1cda0ef")
+        SET(RUBYLIB ${CMAKE_CURRENT_SOURCE_DIR}/win${DFHACK_BUILD_ARCH}/libruby.dll)
+        SET(RUBYLIB_INSTALL_NAME "libruby.dll")
+        IF(${DFHACK_BUILD_ARCH} STREQUAL 64)
+            DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/win64-libruby200.dll.gz"
+                "gz"
+                ${RUBYLIB}.gz
+                "81db54a8b8b3090c94c6ae2147d30b8f"
+                ${RUBYLIB}
+                "8a8564418aebddef3dfee1e96690e713")
+        ELSE()
+            DOWNLOAD_FILE_UNZIP("https://github.com/DFHack/dfhack-bin/releases/download/0.43.05/win32-libruby187.dll.gz"
+                "gz"
+                ${RUBYLIB}.gz
+                "ffc0f1b5b33748e2a36128e90c97f6b2"
+                ${RUBYLIB}
+                "482c1c418f4ee1a5f04203eee1cda0ef")
+        ENDIF()
     ENDIF()
+
 ENDIF()
 
 IF (APPLE OR UNIX)
@@ -73,7 +80,8 @@ ADD_DEPENDENCIES(ruby ruby-autogen-rb)
 IF(EXISTS ${RUBYLIB})
     INSTALL(FILES ${RUBYLIB} DESTINATION ${DFHACK_LIBRARY_DESTINATION} RENAME ${RUBYLIB_INSTALL_NAME})
 ELSE()
-    IF(NOT(APPLE AND ${DFHACK_BUILD_ARCH} STREQUAL 64))
+    # Only fire this warning if DOWNLOAD_RUBY was set.
+    IF(NOT(APPLE AND ${DFHACK_BUILD_ARCH} STREQUAL 64) AND DOWNLOAD_RUBY)
         MESSAGE(WARNING "Ruby library not found at ${RUBYLIB} - will not be installed")
     ENDIF()
 ENDIF()