Use pkg-config for libpq by default

FS-5821 --resolve

configure.in

@@ -419,7 +419,7 @@ SWITCH_AM_CFLAGS="$LIBUUID_CFLAGS $SWITCH_AM_CFLAGS"
 AC_ARG_ENABLE(core-pgsql-support,     
 	[AS_HELP_STRING([--enable-core-pgsql-support], [Compile with PGSQL Support])],,[enable_core_pgsql_support="no"])
 AC_ARG_ENABLE(core-pgsql-pkgconfig,
-	[AS_HELP_STRING([--enable-core-pgsql-pkgconfig], [Use pkg-config to get PGQSL build options])],,[enable_core_pgsql_pkgconfig="no"])
+	[AS_HELP_STRING([--disable-core-pgsql-pkgconfig], [Use pg_config to get PGQSL build options])],[enable_core_pgsql_pkgconfig="$enableval"],[enable_core_pgsql_pkgconfig="yes"])
 
 if test x"$enable_core_pgsql_support" = x"yes" ; then
   AC_PATH_PROG([PG_CONFIG], [pg_config], [no])