libphobos: Skip curl tests if libcurl is not installed on the target.

libphobos: Skip curl tests if libcurl is not installed on the target.

libphobos/ChangeLog:

2019-04-24  Iain Buclaw  <ibuclaw@gdcproject.org>

	PR d/88654
	* testsuite/lib/libphobos.exp (libphobos-dg-test): Check
	libphobos_skipped_test_p before running test.
	(libphobos-dg-prune): New proc.
	(libphobos_init): Set libphobos_skip_tests.
	(libphobos_skipped_test_p): New proc.
	(check_effective_target_libcurl_available): New proc.
	* testsuite/libphobos.phobos/phobos.exp: Skip curl tests if library
	not found.
	* testsuite/libphobos.phobos_shared/phobos_shared.exp: Likewise.

From-SVN: r270545