configure: better CFLAGS checking

needed for compatibility with older gcc.  Also, threw in
some warnings for things I find offensive
(declaration-after-statement, shadow)

git-svn-id: https://svn.musicpd.org/mpd/trunk@7299 09075e82-0dd4-0310-85a5-a0d7c8717e4f

m4/mpd_check_cflag.m4

@@ -0,0 +1,18 @@
+AC_DEFUN([MPD_CHECK_FLAG],[
+  var=`echo "$1" | tr "-" "_"`
+  AC_CACHE_CHECK([whether the C compiler accepts $1],
+    [mpd_check_cflag_$var],[
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$CFLAGS $1"
+    AC_COMPILE_IFELSE([
+     int main(void) { return 0; }
+     ], [ eval "mpd_check_cflag_$var=yes"
+     ], [ eval "mpd_check_cflag_$var=no" ])
+    CFLAGS="$save_CFLAGS"
+  ])
+  if eval "test x`echo '$mpd_check_cflag_'$var` = xyes"
+  then
+    MPD_CFLAGS="$MPD_CFLAGS $1"
+  fi
+  ])
+])