Ticket #2888: cleanups in master branch.

Fix of exec_prefix expansion and usage.

Some distros use autoconf site scripts resourced via $CONFIG_SITE
environment varaible. That scripts are defined variable libexecdir
as

libexecdir='${exec_prefix}/lib'

In this case, EXTHELPERSDIR variable is generated wrongly, because
the 'exec_prefix' variable isn't expanded.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

configure.ac

@@ -424,11 +424,13 @@ dnl ############################################################################
 
 dnl ${prefix} and ${exec_prefix} are undefined here if --prefix is not used in command line
 dnl Let define ${prefix} and ${exec_prefix}
-test "x$prefix" = "xNONE" && prefix=$ac_default_prefix
-test "x$exec_prefix" = "xNONE" && exec_prefix='${prefix}'
+test "x$prefix" = "xNONE" && prefix="$ac_default_prefix"
+test "x$exec_prefix" = "xNONE" && exec_prefix="${prefix}"
 
 if test x${libexecdir} = x'${exec_prefix}/libexec'; then
     EXTHELPERSDIR=${prefix}/libexec/${PACKAGE}/ext.d
+elif test x${libexecdir} = x'${exec_prefix}/lib'; then
+    EXTHELPERSDIR=${prefix}/lib/${PACKAGE}/ext.d
 else
     EXTHELPERSDIR=${libexecdir}/${PACKAGE}/ext.d
 fi