Implement '--enable-src-release-build' option and make src-release.sh use it

Implement '--enable-src-release-build' option and make src-release.sh use it

The generation of snapshots has been broken since we've disable
in-tree builds for GDB.  Given that src-release.sh performs a build
before creating the release tarball, and that this build is performed
in-tree, the solution we found is to implement a new top-level
configure flag called '--enable-src-release-build' which disables the
in-tree build restriction, and then make src-release.sh use it.

ChangeLog:
2020-02-01  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Eli Zaretskii  <eliz@gnu.org>

	* configure.ac: Don't abort the build if trying to build GDB in tree
	_and_ invoking with '--enable-src-release-build'.
	* configure: Regenerate.
	* src-release.sh (do_proto_toplev): Invoke 'configure' using

Change-Id: Ic6dd55accd9a03e62fe498f7fd704fb67f44bfa9