pgsql: Revert to the pre-8.2 method of probing for libm, that is, always

Log Message:
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Revert to the pre-8.2 method of probing for libm, that is, always
include it if it links properly.  It seems too risky to assume that
standard functions like pow() are not special-cased by the compiler.
Per report from Andreas Lange that build fails on Solaris cc compiler
with -fast.  Even though we don't consider that a supported option,
I'm worried that similar issues will arise with other compilers.

Modified Files:
--------------
    pgsql:
        configure.in (r1.485 -> r1.486)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.485&r2=1.486)
        configure (r1.516 -> r1.517)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.516&r2=1.517)
    pgsql/src/include:
        pg_config.h.in (r1.105 -> r1.106)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.105&r2=1.106)