Supporting custom-spec.gmk

[Jeremy Manson]

I note that the bottom of spec.gmk.in has this nugget:

# Include the custom-spec.gmk file if it exists
-include $(dir @SPEC@)/custom-spec.gmk

There is no support in the configuration file to make sure that this file
exists in the build directory.  Any support for something like the
following patch?

diff -r 0cc21882d2f6 common/autoconf/basics.m4
--- a/common/autoconf/basics.m4 Thu Sep 26 10:43:04 2013 -0700
+++ b/common/autoconf/basics.m4 Wed Oct 02 17:19:33 2013 -0700
@@ -457,6 +457,10 @@
 AC_CONFIG_HEADERS([$OUTPUT_ROOT/config.h:$AUTOCONF_DIR/config.h.in])
 # The spec.gmk file contains all variables for the make system.
 AC_CONFIG_FILES([$OUTPUT_ROOT/spec.gmk:$AUTOCONF_DIR/spec.gmk.in])
+# The custom-spec.gmk file is an optional file that contains user-specific
config
+if test -e $AUTOCONF_DIR/custom-spec.gmk.in; then
+  AC_CONFIG_FILES([$OUTPUT_ROOT/custom-spec.gmk:$AUTOCONF_DIR/
custom-spec.gmk.in])
+fi
 # The hotspot-spec.gmk file contains legacy variables for the hotspot make
system.
 AC_CONFIG_FILES([$OUTPUT_ROOT/hotspot-spec.gmk:$AUTOCONF_DIR/
hotspot-spec.gmk.in])
 # The bootcycle-spec.gmk file contains support for boot cycle builds.