Index: configure.ac
==================================================================
--- configure.ac
+++ configure.ac
@@ -235,13 +235,13 @@
 	])
 ])
 
 AX_CHECK_COMPILER_FLAGS(-pipe, [OBJCFLAGS="$OBJCFLAGS -pipe"])
 AX_CHECK_COMPILER_FLAGS(-fno-common, [OBJCFLAGS="$OBJCFLAGS -fno-common"])
-AX_CHECK_COMPILER_FLAGS(-fno-constant-cfstrings, [
-	OBJCFLAGS="$OBJCFLAGS -fno-constant-cfstrings"
-	AC_SUBST(NO_CONST_CFSTRINGS, "-fno-constant-cfstrings")
+AX_CHECK_COMPILER_FLAGS(-Xclang -fno-constant-cfstrings, [
+	OBJCFLAGS="$OBJCFLAGS -Xclang -fno-constant-cfstrings"
+	AC_SUBST(NO_CONST_CFSTRINGS, "-Xclang -fno-constant-cfstrings")
 ])
 AX_CHECK_COMPILER_FLAGS([-Wsign-compare -Werror],
 	[OBJCFLAGS="$OBJCFLAGS -Wsign-compare"])
 AS_IF([test x"$with_nds" != x"yes"], [
 	AX_CHECK_COMPILER_FLAGS([-Wshadow -Werror],
@@ -416,11 +416,11 @@
 			[Whether we use the ObjFW runtime])
 
 		AC_MSG_CHECKING([whether -fobjc-runtime=objfw is supported])
 
 		old_OBJCFLAGS="$OBJCFLAGS"
-		OBJCFLAGS="$OBJCFLAGS -fobjc-runtime=objfw"
+		OBJCFLAGS="$OBJCFLAGS -Xclang -fobjc-runtime=objfw"
 		AC_TRY_LINK([
 			#ifdef __has_attribute
 			# if __has_attribute(objc_root_class)
 			__attribute__((__objc_root_class__))
 			# endif
@@ -446,11 +446,11 @@
 			{
 			}
 		], [
 			[Test test];
 		], [
-			RUNTIME_FLAGS="-fobjc-runtime=objfw"
+			RUNTIME_FLAGS="-Xclang -fobjc-runtime=objfw"
 			AC_MSG_RESULT(yes)
 		], [
 			RUNTIME_FLAGS="-fgnu-runtime"
 			OBJCFLAGS="$old_OBJCFLAGS -fgnu-runtime"
 			AC_MSG_RESULT(no)
@@ -900,12 +900,13 @@
 		AC_MSG_RESULT(WinAPI)
 		;;
 	*)
 		AC_MSG_RESULT(POSIX)
 
-		AX_CHECK_COMPILER_FLAGS(-pthread, [
-			CPPFLAGS="$CPPFLAGS -pthread"
+		dnl Use -Wp, as we only use it for the preprocessor.
+		AX_CHECK_COMPILER_FLAGS(-Wp,-pthread, [
+			CPPFLAGS="$CPPFLAGS -Wp,-pthread"
 		], [
 			CPPFLAGS="$CPPFLAGS -D_REENTRANT -D_THREAD_SAFE"
 		])
 
 		AC_CHECK_LIB(pthread, pthread_create, LIBS="$LIBS -lpthread")
@@ -1479,16 +1480,16 @@
 dnl This needs to be after all other header checks, as they include unistd.h,
 dnl which in old glibc versions uses __block. This is worked around in the code
 dnl by providing a wrapper for unistd.h which takes care of this.
 AC_MSG_CHECKING(whether Objective C compiler supports blocks)
 old_OBJCFLAGS="$OBJCFLAGS"
-OBJCFLAGS="$OBJCFLAGS -fblocks"
+OBJCFLAGS="$OBJCFLAGS -Xclang -fblocks"
 AC_TRY_COMPILE([], [
 	int (^foo)(int bar);
 	foo = ^ (int bar) { return 0; }
 ], [
-	AC_SUBST(BLOCKS_FLAGS, "-fblocks")
+	AC_SUBST(BLOCKS_FLAGS, "-Xclang -fblocks")
 	AC_SUBST(OFBLOCKTESTS_M, "OFBlockTests.m")
 	AC_MSG_RESULT(yes)
 ], [
 	AC_MSG_RESULT(no)
 	OBJCFLAGS="$old_OBJCFLAGS"