CMake: generate SkUserConfig.h with configure_file

cmake/CMakeLists.txt

Index: cmake/CMakeLists.txt
diff --git a/cmake/CMakeLists.txt b/cmake/CMakeLists.txt
index fcbce4ff5c76edda74704bb00ea7952908d75813..adc41501bd14eacd9442b78ec8a9744182c32bbb 100644
--- a/cmake/CMakeLists.txt
+++ b/cmake/CMakeLists.txt
@@ -216,6 +216,7 @@ if (OSMESA_LIBRARIES AND OSMESA_INCLUDE_DIRS)
     list (APPEND libs             ${OSMESA_LIBRARIES})
     list (APPEND private_includes ${OSMESA_INCLUDE_DIRS})
     list (APPEND public_defines   "-DSK_MESA=1")
+    set (SK_MESA 1)
 else()
     remove_srcs(../src/gpu/gl/mesa/*)
 endif()
@@ -261,21 +262,10 @@ install(FILES ${c_headers} DESTINATION include)
 install(TARGETS skia DESTINATION lib)
 
 # SkUserConfig.h
-set (userconfig_path ${userconfig_directory}/SkUserConfig.h)
-file(WRITE ${userconfig_path} "// SkUserConfig generated by CMake.\n")
-file(APPEND ${userconfig_path} "#ifndef SkUserConfig_DEFINED\n")
-file(APPEND ${userconfig_path} "#define SkUserConfig_DEFINED\n")
 if (CMAKE_BUILD_TYPE STREQUAL Release)
-  file(APPEND ${userconfig_path} "#define SK_RELEASE\n")
-  file(APPEND ${userconfig_path} "#undef SK_DEBUG\n")
-else ()
-  file(APPEND ${userconfig_path} "#define SK_DEBUG\n")
-  file(APPEND ${userconfig_path} "#undef SK_RELEASE\n")
-endif ()
-if (OSMESA_LIBRARIES AND OSMESA_INCLUDE_DIRS)
-  file (APPEND ${userconfig_path} "#define SK_MESA 1\n")
+    set (SK_RELEASE 1)
 endif()
-file(APPEND ${userconfig_path} "#endif  // SkUserConfig_DEFINED\n")
+configure_file ("SkUserConfig.h.in" "${userconfig_directory}/SkUserConfig.h")
 
 # skia_link_arguments.txt
 set (link_arguments ${CMAKE_BINARY_DIR}/skia_link_arguments.txt)