Add a configure check for the pch flags as clang doesn't support -fpch-deps

Originally committed to SVN as r6918.

aegisub/Makefile.target

@@ -147,7 +147,7 @@ OBJCXXFLAGS := $(CXXFLAGS)
 ####################
 ifeq (yes, $(PRECOMPILED_HEADER))
   ifdef PRECOMPILED_HEADER_NAME
-    CXXFLAGS += -include $(PRECOMPILED_HEADER_NAME) -Winvalid-pch -fpch-deps -fpch-preprocess
+    CXXFLAGS += -include $(PRECOMPILED_HEADER_NAME) $(PCHFLAGS)
     # This hack is required until we can use the 'private' keyword (gmake 3.82) to avoid all the
     # per-object CXXFLAGS being triggered by the ($OBJ) line.
     CXXFLAGS_GNU_MAKE_SUCKS := $(CXXFLAGS)

aegisub/configure.in

@@ -532,6 +532,9 @@ AC_MSG_CHECKING([whether to use precompiled headers])
 AC_ARG_ENABLE(gcc-prec, AS_HELP_STRING([--disable-gcc-prec],[disable GCC precompiled headers]))
 AC_MSG_RESULT([${enable_gcc_prec:=yes}])
 AC_SUBST(enable_gcc_prec)
+AC_PCH_FLAG([-Winvalid-pch])
+AC_PCH_FLAG([-fpch-deps])
+AC_PCH_FLAG([-fpch-preprocess])
 
 ###########
 # Profiling

aegisub/m4macros/ac_flag.m4

@@ -28,3 +28,24 @@ AC_DEFUN([AC_CXX_FLAG], [{
 		])
 	AC_LANG_POP(C++)
 	}])
+AC_DEFUN([AC_PCH_FLAG], [{
+	AC_LANG_PUSH(C++)
+	ac_cxx_flag_save="$CXXFLAGS"
+	ac_cxx_werror_flag_save="$ac_cxx_werror_flag"
+	ac_cxx_werror_flag=yes
+	CXXFLAGS="$CXXFLAGS -Werror $1"
+	AC_MSG_CHECKING([[whether $CXX supports $1]])
+	AC_COMPILE_IFELSE(
+		[AC_LANG_PROGRAM([[]])],
+		[
+			PCHFLAGS="$PCHFLAGS $1"
+			AC_MSG_RESULT([yes])
+		],
+		[
+			AC_MSG_RESULT([no])
+			$2
+		])
+	CXXFLAGS="$ac_cxx_flag_save"
+	ac_cxx_werror_flag="$ac_cxx_werror_flag_save"
+	AC_LANG_POP(C++)
+	}])