pgsql: Replace obsolete AC_LANG_FUNC_LINK_TRY autoconf macro.

Replace obsolete AC_LANG_FUNC_LINK_TRY autoconf macro.

The version of this macro used in autoconf 2.59 is capable of incorrectly
succeeding (ie, reporting that a library function is available when it
isn't), if the compiler performs link-time optimization and decides that
it can optimize the function reference away entirely.  Replace it with the
coding used in autoconf 2.61 and later, which forces the program result to
depend on the function's result so that it cannot be optimized away.  This
should fix build failures currently being seen on buildfarm member anchovy.

This patch affects the 8.2 and 8.3 branches only, since later branches are
using autoconf versions that don't have this problem.

Branch
------
REL8_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/cbd51548dc1869fde58b621ea3c2fc9386947ea9

Modified Files
--------------
config/c-library.m4 |   69 +++++++
configure           |  554 +++++++++++++++++++++------------------------------
2 files changed, 298 insertions(+), 325 deletions(-)