package/php: fix build without threads and pcre

The pcre library bundled in php is not configurable through the usual
--enable/disable options, but wants to be thread-safe by default, so it
wants to use pthreads (uncludes pthread.h and uses pthread_* functions).

So, just override the default configuration to explicitly require the
bundled pcre library be single-threaded when we have no thread and are
not using the external pcre library.

Fixes:
    http://autobuild.buildroot.org/results/f50/f50f4e569466ad74e49a3eac9ea2e6cfc4dd30e3/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Floris Bos <bos@je-eigen-domein.nl>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>