Add warnings m4 macros from GNOME

This is better than what we've got currently, and
even works with older versions of GCC.

acinclude.m4

@@ -39,3 +39,144 @@ AC_DEFUN([AS_AC_EXPAND],
   exec_prefix=$exec_prefix_save
 ])
 
+dnl GNOME_COMPILE_WARNINGS
+dnl Turn on many useful compiler warnings
+dnl For now, only works on GCC
+AC_DEFUN([GNOME_COMPILE_WARNINGS],[
+    dnl ******************************
+    dnl More compiler warnings
+    dnl ******************************
+
+    AC_ARG_ENABLE(compile-warnings, 
+                  AC_HELP_STRING([--enable-compile-warnings=@<:@no/minimum/yes/maximum/error@:>@],
+                                 [Turn on compiler warnings]),,
+                  [enable_compile_warnings="m4_default([$1],[yes])"])
+
+    warnCFLAGS=
+    if test "x$GCC" != xyes; then
+	enable_compile_warnings=no
+    fi
+
+    warning_flags=
+    realsave_CFLAGS="$CFLAGS"
+
+    case "$enable_compile_warnings" in
+    no)
+	warning_flags=
+	;;
+    minimum)
+	warning_flags="-Wall"
+	;;
+    yes)
+	warning_flags="-Wall -Wmissing-prototypes"
+	;;
+    maximum|error)
+	warning_flags="-Wall -Wmissing-prototypes -Wnested-externs -Wpointer-arith"
+	CFLAGS="$warning_flags $CFLAGS"
+	for option in -Wno-sign-compare; do
+		SAVE_CFLAGS="$CFLAGS"
+		CFLAGS="$CFLAGS $option"
+		AC_MSG_CHECKING([whether gcc understands $option])
+		AC_TRY_COMPILE([], [],
+			has_option=yes,
+			has_option=no,)
+		CFLAGS="$SAVE_CFLAGS"
+		AC_MSG_RESULT($has_option)
+		if test $has_option = yes; then
+		  warning_flags="$warning_flags $option"
+		fi
+		unset has_option
+		unset SAVE_CFLAGS
+	done
+	unset option
+	if test "$enable_compile_warnings" = "error" ; then
+	    warning_flags="$warning_flags -Werror"
+	fi
+	;;
+    *)
+	AC_MSG_ERROR(Unknown argument '$enable_compile_warnings' to --enable-compile-warnings)
+	;;
+    esac
+    CFLAGS="$realsave_CFLAGS"
+    AC_MSG_CHECKING(what warning flags to pass to the C compiler)
+    AC_MSG_RESULT($warning_flags)
+
+    AC_ARG_ENABLE(iso-c,
+                  AC_HELP_STRING([--enable-iso-c],
+                                 [Try to warn if code is not ISO C ]),,
+                  [enable_iso_c=no])
+
+    AC_MSG_CHECKING(what language compliance flags to pass to the C compiler)
+    complCFLAGS=
+    if test "x$enable_iso_c" != "xno"; then
+	if test "x$GCC" = "xyes"; then
+	case " $CFLAGS " in
+	    *[\ \	]-ansi[\ \	]*) ;;
+	    *) complCFLAGS="$complCFLAGS -ansi" ;;
+	esac
+	case " $CFLAGS " in
+	    *[\ \	]-pedantic[\ \	]*) ;;
+	    *) complCFLAGS="$complCFLAGS -pedantic" ;;
+	esac
+	fi
+    fi
+    AC_MSG_RESULT($complCFLAGS)
+
+    WARN_CFLAGS="$warning_flags $complCFLAGS"
+    AC_SUBST(WARN_CFLAGS)
+])
+
+dnl For C++, do basically the same thing.
+
+AC_DEFUN([GNOME_CXX_WARNINGS],[
+  AC_ARG_ENABLE(cxx-warnings,
+                AC_HELP_STRING([--enable-cxx-warnings=@<:@no/minimum/yes@:>@]
+                               [Turn on compiler warnings.]),,
+                [enable_cxx_warnings="m4_default([$1],[minimum])"])
+
+  AC_MSG_CHECKING(what warning flags to pass to the C++ compiler)
+  warnCXXFLAGS=
+  if test "x$GXX" != xyes; then
+    enable_cxx_warnings=no
+  fi
+  if test "x$enable_cxx_warnings" != "xno"; then
+    if test "x$GXX" = "xyes"; then
+      case " $CXXFLAGS " in
+      *[\ \	]-Wall[\ \	]*) ;;
+      *) warnCXXFLAGS="-Wall -Wno-unused" ;;
+      esac
+
+      ## -W is not all that useful.  And it cannot be controlled
+      ## with individual -Wno-xxx flags, unlike -Wall
+      if test "x$enable_cxx_warnings" = "xyes"; then
+	warnCXXFLAGS="$warnCXXFLAGS -Wshadow -Woverloaded-virtual"
+      fi
+    fi
+  fi
+  AC_MSG_RESULT($warnCXXFLAGS)
+
+   AC_ARG_ENABLE(iso-cxx,
+                 AC_HELP_STRING([--enable-iso-cxx],
+                                [Try to warn if code is not ISO C++ ]),,
+                 [enable_iso_cxx=no])
+
+   AC_MSG_CHECKING(what language compliance flags to pass to the C++ compiler)
+   complCXXFLAGS=
+   if test "x$enable_iso_cxx" != "xno"; then
+     if test "x$GXX" = "xyes"; then
+      case " $CXXFLAGS " in
+      *[\ \	]-ansi[\ \	]*) ;;
+      *) complCXXFLAGS="$complCXXFLAGS -ansi" ;;
+      esac
+
+      case " $CXXFLAGS " in
+      *[\ \	]-pedantic[\ \	]*) ;;
+      *) complCXXFLAGS="$complCXXFLAGS -pedantic" ;;
+      esac
+     fi
+   fi
+  AC_MSG_RESULT($complCXXFLAGS)
+
+  WARN_CXXFLAGS="$CXXFLAGS $warnCXXFLAGS $complCXXFLAGS"
+  AC_SUBST(WARN_CXXFLAGS)
+])