cmake: honor CMAKE_FLAGS environment variable

Expose `CMAKE_FLAGS` so that users can supply configuration flags to
cmake (eg `-DUSE_SHA1DC=ON`)

Author: Edward Thomson

ext/rugged/extconf.rb

@@ -77,7 +77,7 @@ def sys(cmd)
     Dir.chdir("build") do
       # On Windows, Ruby-DevKit is MSYS-based, so ensure to use MSYS Makefiles.
       generator = "-G \"MSYS Makefiles\"" if Gem.win_platform?
-      sys("cmake .. -DBUILD_CLAR=OFF -DTHREADSAFE=ON -DBUILD_SHARED_LIBS=OFF -DCMAKE_C_FLAGS=-fPIC -DCMAKE_BUILD_TYPE=RelWithDebInfo #{generator}")
+      sys("cmake .. -DBUILD_CLAR=OFF -DTHREADSAFE=ON -DBUILD_SHARED_LIBS=OFF -DCMAKE_C_FLAGS=-fPIC -DCMAKE_BUILD_TYPE=RelWithDebInfo #{ENV['CMAKE_FLAGS']} #{generator}")
       sys(MAKE)
 
       # "normal" libraries (and libgit2 builds) get all these when they build but we're doing it