toggle the default for this option

configure.in

@@ -5511,11 +5511,8 @@ dnl ===================================================================
 dnl Check for system Mesa
 dnl ===================================================================
 AC_MSG_CHECKING([which Mesa headers to use])
-if test "$with_system_mesa_headers" = "no"; then
-   AC_MSG_RESULT([internal])
-   SYSTEM_MESA_HEADERS=NO
-   BUILD_TYPE="$BUILD_TYPE MESA"
-else
+if test -n "$with_system_mesa_headers" -o -n "$with_system_headers" && \
+    test "$with_system_mesa_headers" != "no"; then
    AC_MSG_RESULT([external])
    SYSTEM_MESA_HEADERS=YES
    AC_LANG_PUSH(C)
@@ -5524,6 +5521,10 @@ else
    AC_MSG_CHECKING([whether GL/glxext.h defines PFNGLXBINDTEXIMAGEEXTPROC])
    AC_EGREP_HEADER(PFNGLXBINDTEXIMAGEEXTPROC, GL/glxext.h, [AC_MSG_RESULT([yes])], AC_MSG_ERROR(no))
    AC_LANG_POP(C)
+else
+   AC_MSG_RESULT([internal])
+   SYSTEM_MESA_HEADERS=NO
+   BUILD_TYPE="$BUILD_TYPE MESA"
 fi
 AC_SUBST(SYSTEM_MESA_HEADERS)