diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -196,6 +196,7 @@
 
 option(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT "Enable per TU ABI insulation by default. To be used by vendors." OFF)
 set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros to define in the site config header.")
+option(LIBCXX_EXTRA_SITE_DEFINES "Extra defines to add into __config_site")
 option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 set(LIBCXX_LIBCPPABI_VERSION "2" CACHE STRING "Version of libc++abi's ABI to re-export from libc++ when re-exporting is enabled.
                                                Note that this is not related to the version of libc++'s ABI itself!")
@@ -909,6 +910,17 @@
   config_define(${abi_defines} _LIBCPP_ABI_DEFINES)
 endif()
 
+if (LIBCXX_EXTRA_SITE_DEFINES)
+  set(extra_site_defines)
+  foreach (extra_site_define ${LIBCXX_EXTRA_SITE_DEFINES})
+    # Allow defines such as DEFINE=VAL, transformed into "#define DEFINE VAL".
+    string(REPLACE "=" " " extra_site_define "${extra_site_define}")
+    list(APPEND extra_site_defines "#define ${extra_site_define}")
+  endforeach()
+  string(REPLACE ";" "\n" extra_site_defines "${extra_site_defines}")
+  config_define(${extra_site_defines} _LIBCPP_EXTRA_SITE_DEFINES)
+endif()
+
 # By default libc++ on Windows expects to use a shared library, which requires
 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
diff --git a/libcxx/cmake/caches/MinGW.cmake b/libcxx/cmake/caches/MinGW.cmake
--- a/libcxx/cmake/caches/MinGW.cmake
+++ b/libcxx/cmake/caches/MinGW.cmake
@@ -13,8 +13,6 @@
 
 set(LIBCXX_TARGET_INFO "libcxx.test.target_info.MingwLocalTI" CACHE STRING "")
 
-# Without these flags, 'long double' (which is 80 bit on x86 mingw, but
+# Without this flag, 'long double' (which is 80 bit on x86 mingw, but
 # 64 bit in MSVC) isn't handled correctly in printf.
-set(CMAKE_C_FLAGS "-D__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")
-set(CMAKE_CXX_FLAGS "-D__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")
-set(LIBCXX_TEST_COMPILER_FLAGS "-D__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")
+set(LIBCXX_EXTRA_SITE_DEFINES "__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -35,5 +35,6 @@
 #cmakedefine _LIBCPP_HAS_NO_INCOMPLETE_RANGES
 
 @_LIBCPP_ABI_DEFINES@
+@_LIBCPP_EXTRA_SITE_DEFINES@
 
 #endif // _LIBCPP_CONFIG_SITE
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -108,8 +108,7 @@
           -DCMAKE_C_COMPILER=clang-cl \
           -DCMAKE_CXX_COMPILER=clang-cl \
           -DLIBCXX_ENABLE_FILESYSTEM=YES \
-          -DCMAKE_CXX_FLAGS="-D_LIBCPP_HAS_NO_INT128" \
-          -DLIBCXX_TEST_COMPILER_FLAGS="-D_LIBCPP_HAS_NO_INT128" \
+          -DLIBCXX_EXTRA_SITE_DEFINES="_LIBCPP_HAS_NO_INT128" \
           "${@}"
 }