Starting with glibc 2.25, the proc_service.h header has been copied
from gdb to glibc so other tools can use it. However, that makes it
necessary to make sure that declaration of prfpregset_t declaration
is consistent between gdb and glibc. In gdb, however, there is a
workaround for a broken prfpregset_t declaration in glibc 2.3 which
uses AC_TRY_RUN to detect if it's needed, which doesn't work in
cross-compilation. So pass the cache option to configure.
It needs to be passed to GDB_CONF_ENV to build gdbserver only but
also to GDB_MAKE_ENV, because otherwise it does not get passed to the
configure script of nested packages while building gdbserver with full
debugger.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>