bootstrap: accept --gnulib-srcdir=/path/to/gnulib again.

During the split of bootstrap into options-parser and
extract-trace, the generic long-option splitting code got lost.
* libltdl/config/options-parser (func_parse_options): Add the
code to split any long option separated by an equals from its
optarg back in.
Reported by Roumen Petrov.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>

libltdl/config/options-parser

@@ -436,6 +436,13 @@ func_parse_options ()
         -\?|-h)       func_usage ;;
         --help)       func_help ;;
 
+	# Separate optargs to long options (plugins may need this):
+	--*=*)        func_split_equals "$opt"
+	              set dummy "$func_split_equals_lhs" \
+                          "$func_split_equals_rhs" ${1+"$@"}
+                      shift
+                      ;;
+
         # Separate non-argument short options:
         -\?*|-h*|-v*|-x*)
                       func_split_short_opt "$_G_opt"