package/postgresql: fix pthread flags detection

PostgreSQL configure may not correctly determine flags for compiling and
linking with pthreads support when compiler or linker give irrelevant
warnings.

Record default compiler and linker output and see if adding pthread
option changes that, instead of assuming that linker and compiler are
silent by default.

Fixes:
  http://autobuild.buildroot.net/results/916487aae929c181d4e3f471c6c7508552f0b683/
  http://autobuild.buildroot.net/results/921a94d1a489d81b362c5b6a403a156b4691effd/
  http://autobuild.buildroot.net/results/dbcdc58ad32b7b811ea47b6fc0f926a477e30786/
  http://autobuild.buildroot.net/results/68b582f4caec223a646f095487263353f273156d/



Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>