CMake: Only set CMAKE_BUILD_TYPE_INIT when not set

Convenience makefile now uses CMAKE_BUILD_TYPE_INIT,
this means you can change the build type of an existing build
and it won't be overwritten when running `make`.

Useful if you want to add debug info to a release build for profiling.

GNUmakefile

@@ -104,7 +104,7 @@ endif
 CMAKE_CONFIG = cmake $(BUILD_CMAKE_ARGS) \
                      -H"$(BLENDER_DIR)" \
                      -B"$(BUILD_DIR)" \
-                     -DCMAKE_BUILD_TYPE:STRING=$(BUILD_TYPE)
+                     -DCMAKE_BUILD_TYPE_INIT:STRING=$(BUILD_TYPE)
 
 
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