Fix the Mac OS X SDK configure check

configure.ac

@@ -247,7 +247,7 @@ dnl  Check for Mac OS X SDK settings
 dnl
 AC_ARG_WITH(macosx-sdk,
    [  --with-macosx-sdk       compile for Mac OS X 10.3 SDK],[],[])
-if test -n "${with_macosx-sdk}"; then
+if test "${with_macosx-sdk}" = "yes" ; then
   CFLAGS_save="${CFLAGS_save} -DMACOSX_DEPLOYMENT_TARGET=10.3 -isysroot /Developer/SDKs/MacOSX10.3.9.sdk"; CFLAGS="${CFLAGS_save}"
   CXXFLAGS_save="${CXXFLAGS_save} -DMACOSX_DEPLOYMENT_TARGET=10.3 -isysroot /Developer/SDKs/MacOSX10.3.9.sdk"; CXXFLAGS="${CXXFLAGS_save}"
   OBJCFLAGS_save="${OBJCFLAGS_save} -DMACOSX_DEPLOYMENT_TARGET=10.3 -isysroot /Developer/SDKs/MacOSX10.3.9.sdk"; OBJCFLAGS="${OBJCFLAGS_save}"