CMAKE_BUILD_TYPE should not get overwriten if empty

[FFY00]

As documented, CMAKE_BUILD_TYPE being empty is a valid value with its own behavior (do not touch the compile flags, just use the CFLAGS/CXXFLAGS/etc specified in the environment). libgit2 is overwriting the option to Debug if empty, changing the expected behavior.
Would it be possible for you to consider removing this?

libgit2/CMakeLists.txt

Lines 254 to 256 in e167476

	IF (NOT CMAKE_BUILD_TYPE)
	SET(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Choose the type of build, options are: Debug Release RelWithDebInfo MinSizeRel." FORCE)
	ENDIF ()