--- a/build/autoconf/clang-plugin.m4
+++ b/build/autoconf/clang-plugin.m4
@@ -99,26 +99,29 @@ if test -n "$ENABLE_CLANG_PLUGIN"; then
     dnl middle of the 3.8 cycle, our CLANG_VERSION_FULL is impossible to use
     dnl correctly, so we have to detect this at configure time.
     AC_CACHE_CHECK(for new ASTMatcher API,
                    ac_cv_have_new_ASTMatcher_api,
         [
             AC_LANG_SAVE
             AC_LANG_CPLUSPLUS
             _SAVE_CXXFLAGS="$CXXFLAGS"
+            _SAVE_CPPFLAGS="$CPPFLAGS"
             _SAVE_CXX="$CXX"
             _SAVE_MACOSX_DEPLOYMENT_TARGET="$MACOSX_DEPLOYMENT_TARGET"
             unset MACOSX_DEPLOYMENT_TARGET
             CXXFLAGS="${LLVM_CXXFLAGS}"
+            CPPFLAGS=""
             CXX="${HOST_CXX}"
             AC_TRY_COMPILE([#include "clang/ASTMatchers/ASTMatchers.h"],
                            [clang::ast_matchers::cxxConstructExpr();],
                            ac_cv_have_new_ASTMatcher_names="yes",
                            ac_cv_have_new_ASTMatcher_names="no")
             CXX="$_SAVE_CXX"
+            CPPFLAGS="$_SAVE_CPPFLAGS"
             CXXFLAGS="$_SAVE_CXXFLAGS"
             export MACOSX_DEPLOYMENT_TARGET="$_SAVE_MACOSX_DEPLOYMENT_TARGET"
             AC_LANG_RESTORE
         ])
     if test "$ac_cv_have_new_ASTMatcher_names" = "yes"; then
       LLVM_CXXFLAGS="$LLVM_CXXFLAGS -DHAVE_NEW_ASTMATCHER_NAMES"
     fi
 
@@ -126,28 +129,31 @@ if test -n "$ENABLE_CLANG_PLUGIN"; then
     dnl before 3.9 that ignoringParenImpCasts was done internally by "has".
     dnl See https://www.mail-archive.com/cfe-commits@lists.llvm.org/msg25234.html
     AC_CACHE_CHECK(for has with ignoringParenImpCasts,
                    ac_cv_has_accepts_ignoringParenImpCasts,
         [
             AC_LANG_SAVE
             AC_LANG_CPLUSPLUS
             _SAVE_CXXFLAGS="$CXXFLAGS"
+            _SAVE_CPPFLAGS="$CPPFLAGS"
             _SAVE_CXX="$CXX"
             _SAVE_MACOSX_DEPLOYMENT_TARGET="$MACOSX_DEPLOYMENT_TARGET"
             unset MACOSX_DEPLOYMENT_TARGET
             CXXFLAGS="${LLVM_CXXFLAGS}"
+            CPPFLAGS=""
             CXX="${HOST_CXX}"
             AC_TRY_COMPILE([#include "clang/ASTMatchers/ASTMatchers.h"],
                            [using namespace clang::ast_matchers;
                             expr(has(ignoringParenImpCasts(declRefExpr())));
                            ],
                            ac_cv_has_accepts_ignoringParenImpCasts="yes",
                            ac_cv_has_accepts_ignoringParenImpCasts="no")
             CXX="$_SAVE_CXX"
+            CPPFLAGS="$_SAVE_CPPFLAGS"
             CXXFLAGS="$_SAVE_CXXFLAGS"
             export MACOSX_DEPLOYMENT_TARGET="$_SAVE_MACOSX_DEPLOYMENT_TARGET"
             AC_LANG_RESTORE
         ])
     if test "$ac_cv_has_accepts_ignoringParenImpCasts" = "yes"; then
       LLVM_CXXFLAGS="$LLVM_CXXFLAGS -DHAS_ACCEPTS_IGNORINGPARENIMPCASTS"
     fi