Prioritize using the header files of self (#4116)

CMakeLists.txt

@@ -319,7 +319,7 @@ else ()
   message(WARNING "Feature cxx_std_11 is unknown for the CXX compiler")
 endif ()
 
-target_include_directories(fmt ${FMT_SYSTEM_HEADERS_ATTRIBUTE} PUBLIC
+target_include_directories(fmt ${FMT_SYSTEM_HEADERS_ATTRIBUTE} BEFORE PUBLIC
   $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
   $<INSTALL_INTERFACE:${FMT_INC_DIR}>)
 
@@ -369,7 +369,7 @@ target_compile_definitions(fmt-header-only INTERFACE FMT_HEADER_ONLY=1)
 target_compile_features(fmt-header-only INTERFACE cxx_std_11)
 
 target_include_directories(fmt-header-only
-  ${FMT_SYSTEM_HEADERS_ATTRIBUTE} INTERFACE
+  ${FMT_SYSTEM_HEADERS_ATTRIBUTE} BEFORE INTERFACE
   $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
   $<INSTALL_INTERFACE:${FMT_INC_DIR}>)