c11/threads: resolve link issues with -O0

c11/threads: resolve link issues with -O0

Add weak symbol notation for the pthread_mutexattr* symbols, thus making
the linker happy. When building with -O1 or greater the optimiser will
kick in and remove the said functions as they are dead/unreachable code.

Ideally we'll enable the optimisations locally, yet that does not seem
to work atm.

v2: Add the AX_GCC_FUNC_ATTRIBUTE([weak]) hunk in configure.

Cc: Alejandro Piñeiro <apinheiro@igalia.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>