testsuite: Disable ccache

testsuite: Disable ccache

There were always various problems with compatibility with ccache:
	https://bugzilla.redhat.com/show_bug.cgi?id=488863
	https://bugzilla.redhat.com/show_bug.cgi?id=759592
	https://sourceware.org/ml/gdb-patches/2009-02/msg00397.html

IMO in a summary ccache finds more a benefit of faster compilation despite the
debug info is no longer exactly the same (as without ccache).

Although for example in this case ccache helped to find a real GDB bug:
	https://sourceware.org/ml/gdb-patches/2015-01/msg00497.html

For the GDB testcases ccache has (IMO) no real performance advantage and it
just brings heisenbugs - false FAILs - from time to time:

Breakpoint 1, main () at gdb/testsuite/gdb.base/vdso-warning.c:21^M
	21        return 0;^M
	(gdb) PASS: gdb.base/vdso-warning.exp: run: startup
	->
	Breakpoint 1, main () at gdb/testsuite/gdb.base/hbreak-unmapped.c:21^M
	21        return 0;^M
	(gdb) FAIL: gdb.base/vdso-warning.exp: run: startup

So I find most safe and easy to just disable ccache for all testsuites.

gdb/testsuite/ChangeLog
2016-09-15  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* lib/future.exp: Set CCACHE_DISABLE, clear CCACHE_NODISABLE.