Win32: Fix macros being enabled for other backends

_UNICODE should not be defined for other backends even when built on
Windows.

(cherry picked from commit a646f51bca867e04254c5fdcb741741265bc4d6c)

src/CMakeLists.txt

@@ -131,7 +131,7 @@ elseif (CMAKE_C_COMPILER_ID STREQUAL "GNU" OR
     target_compile_options(glfw PRIVATE "-Wall")
 endif()
 
-if (WIN32)
+if (_GLFW_WIN32)
     target_compile_definitions(glfw PRIVATE _UNICODE)
 endif()