BUG #6759: configure script fails to detect xlc compiler version

The following bug has been logged on the website:

Bug reference: 6759
Logged by: jez wain
Email address: jez(dot)wain(at)bull(dot)net
PostgreSQL version: 9.1.4
Operating system: AIX 7.1
Description:

The configure script (near line 3247) detects the compiler version with

--version, -v, and -V

IBM's xlc compiler uses "-qversion" flag to display its version.

This produces:
bash-4.2$ xlc -qversion
IBM XL C/C++ for AIX, V11.1 (5724-X13)
Version: 11.01.0000.0000

If you require additional information, there is a verbose switch,
-qversion=verbose, which produces:

bash-4.2$ xlc -qversion=verbose
IBM XL C/C++ for AIX, V11.1 (5724-X13)
Version: 11.01.0000.0000
Driver Version: 11.01(C/C++) Level: 100304
C Front End Version: 11.00(C/C++) Level: 100301
High-Level Optimizer Version: 11.01(C/C++) and 13.01(Fortran) Level: 100301
Low-Level Optimizer Version: 11.01(C/C++) and 13.01(Fortran) Level: 100304
bash-4.2$

When faced with a flag it doesn't recognize, xlc prints the man page. Thus
the --version, -v, -V tests result in a particularly large config.log file