Add conditional build of subsystems to configure.ac

The toplevel Makefile still works without running configure
and will default build everything

Signed-off-by: Matthew Fioravante <matthew.fioravante@jhuapl.edu>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

m4/subsystem.m4

@@ -0,0 +1,32 @@
+AC_DEFUN([AX_SUBSYSTEM_DEFAULT_ENABLE], [
+AC_ARG_ENABLE([$1],
+AS_HELP_STRING([--disable-$1], [Disable build and install of $1]),[
+$1=n
+],[
+$1=y
+SUBSYSTEMS="$SUBSYSTEMS $1"
+AS_IF([test -e "$1/configure"], [
+AC_CONFIG_SUBDIRS([$1])
+])
+])
+AC_SUBST($1)
+])
+
+AC_DEFUN([AX_SUBSYSTEM_DEFAULT_DISABLE], [
+AC_ARG_ENABLE([$1],
+AS_HELP_STRING([--enable-$1], [Enable build and install of $1]),[
+$1=y
+SUBSYSTEMS="$SUBSYSTEMS $1"
+AS_IF([test -e "$1/configure"], [
+AC_CONFIG_SUBDIRS([$1])
+])
+],[
+$1=n
+])
+AC_SUBST($1)
+])
+
+
+AC_DEFUN([AX_SUBSYSTEM_FINISH], [
+AC_SUBST(SUBSYSTEMS)
+])