gdb/configure.ac: add --enable-source-highlight

gdb/configure.ac: add --enable-source-highlight

Allow disabling source-highlight dependency autodetection even
it exists in the system. More details on problem of automatic
dependencies:
https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Automagic_dependencies

Noticed by Jeroen Roovers in https://bugs.gentoo.org/680238

	* configure.ac: add --enable-source-highlight switch.
	* configure: Regenerate.
	* top.c (print_gdb_version): plumb --enable-source-highlight
	status to "show configuration".

gdb/ChangeLog
2019-04-19  Sergei Trofimovich <siarheit@google.com>

	* configure.ac: add --enable-source-highlight switch.
	* configure: Regenerate.
	* top.c (print_gdb_version): plumb --enable-source-highlight
	status to "show configuration".