CMake: Avoid call of find_package(Git) for every compile with buildinfo enabled

This is a bit tricky, find_package(Git) was only used to check if git is installed
and actual git command was assumed to be in the PATH (while ideally it should have
been GIT_COMMAND variable).

This commit makes re-compile output cleaner, especially when using Ninja.

Adding proper GIT_COMMAND to buildinfo.cmake is also possible via argument.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D1847

CMakeLists.txt

@@ -704,6 +704,13 @@ else()
 	set(COMPILER_SSE2_FLAG)
 endif()
 
+if(WITH_BUILDINFO)
+	find_package(Git)
+	if(NOT GIT_FOUND)
+		set(WITH_BUILDINFO OFF)
+	endif()
+endif()
+
 TEST_SHARED_PTR_SUPPORT()
 TEST_UNORDERED_MAP_SUPPORT()