Ensure BX_DIR is converted to cmake style path

When using the BX_DIR, BIMG_DIR, and BGFX_DIR options to cmake, the
pathnames need to be converted to cmake style path. This ensures the
paths do not have mixed directory separators later on in the build,
which can cause build failures.

CMakeLists.txt

@@ -94,6 +94,10 @@ elseif(NOT IS_ABSOLUTE "${BGFX_DIR}")
 	get_filename_component(BGFX_DIR "${BGFX_DIR}" REALPATH BASE_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
 endif()
 
+file(TO_CMAKE_PATH "${BX_DIR}" BX_DIR)
+file(TO_CMAKE_PATH "${BIMG_DIR}" BIMG_DIR)
+file(TO_CMAKE_PATH "${BGFX_DIR}" BGFX_DIR)
+
 # sets project version from api ver / git rev
 include(${CMAKE_CURRENT_SOURCE_DIR}/cmake/version.cmake)
 if(BGFX_BUILD_TOOLS AND BGFX_CUSTOM_TARGETS)