pgsql: Move pthread-tests earlier in the autoconf script.

Move pthread-tests earlier in the autoconf script.

On some Linux systems, "-lrt" exposed pthread-functions, so that linking
with -lrt was seemingly enough to make a program that uses pthreads to
work. However, when linking libpq, the dependency to libpthread was not
marked correctly, so that when an executable was linked with -lpq but
without -pthread, you got errors about undefined pthread_* functions from
libpq.

To fix, test for the flags required to use pthreads earlier in the autoconf
script, before checking any other libraries.

This should fix the failure on buildfarm member shearwater. gharial is also
failing; hopefully this fixes that too although the failure looks somewhat
different.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/080c4dab3d9575449b81604051b160597cfd55c3

Modified Files
--------------
configure    | 7648 +++++++++++++++++++++++++++++-----------------------------
configure.in |  147 +-
2 files changed, 3900 insertions(+), 3895 deletions(-)