BUG #6758: ./configure script sets HAVE_WCSTOMBS_L 1

The following bug has been logged on the website:

Bug reference: 6758
Logged by: jez wain
Email address: jez(dot)wain(at)bull(dot)net
PostgreSQL version: 9.1.4
Operating system: AIX 7.1
Description:

Building Postgresql on AIX7.1 with IBM's xlc compiler I got an undefined
symbol error on the link phase for the 'postgres' binary:

ld: 0711-317 ERROR: Undefined symbol: .mbstowcs_l
ld: 0711-345 Use the -bloadmap or -bnoquiet option to obtain more
information.

AIX7 supports the mbstowcs API but not mbstowcs_l. In
src/include/pg_config.h I commented out the define for HAVE_WCSTOMBS_L 1,
recompiled src/backend/utils/adt/pg_locale.c and the build completed
successfully.

This indicates that the ./configure script does not correctly detect the
absence of the locale specific wide-character/multi-byte API's (xxxx_l), on
AIX (near line 18945)