ext: Provide post_config hook

Add a post-config hook for MPI extensions, similar to the same
functionality in the MCA system.  The purpose of the post-config
hook is to provie a macro which is executed after configure has
decided whether or not to build an extension.  This is useful for
AC_DEFINEs or AM_CONDITIONALs.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>
(cherry picked from commit 790f4a3)

Author: bwbarrett

config/ompi_ext.m4

@@ -390,6 +390,9 @@ AC_DEFUN([EXT_CONFIGURE_M4_CONFIG_COMPONENT],[
     AS_IF([test $should_build -eq 1],
           [EXT_PROCESS_COMPONENT([$1], [$2], [$3], [$4], [$5], [$6], [$7])],
           [EXT_PROCESS_DEAD_COMPONENT([$1], [$2])])
+
+    m4_ifdef([OMPI_MPIEXT_$1_POST_CONFIG],
+             [OMPI_MPIEXT_$1_POST_CONFIG($should_build)])
 ])
 
 ######################################################################

ompi/mpiext/example/configure.m4

@@ -55,3 +55,12 @@ AC_DEFUN([OMPI_MPIEXT_example_NEED_INIT], [1])
 # libraries for any of the bindings.  If finer-grained control is needed
 # someday, we may need to split this into multiple macros.
 #AC_DEFUN([OMPI_MPIEXT_example_HAVE_OBJECT], [0])
+
+# This is a hook that runs after the CONFIG macro and after the
+# configure system decides if this extension will be built (such
+# as if it was disabled by configure option).  This is a great
+# place to put any globally-visible AC_DEFINE or AM_CONDITIONAL
+# macros that are based on this component building.
+AC_DEFUN([OMPI_MPIEXT_example_POST_CONFIG], [
+  opal_show_verbose "example component build result: $1"
+])