configure tests for the __builtin_math functions

[Steven King <sxking at uswest dot net>]

  The configure checks for the __builtin_math functions are non-functional. 
They do an AC_TRY_COMPILE with AC_LANG_C which will not fail for undefined
functions and thus the tests always succeed.  But this doesnt matter as they
define _GLIBCPP_* for success and the header files are checking for GLIBCPP (no
leading underscore) so the __builtins are being not used.  However, there is a
comment in the Changelog from last year saying the builtins are disabled
because of problems on Solaris so I am not sure if all this is by design.

Now my question is, I've started to add long double support to the library
and when adding configure checks to see what functionality is provided by the
compiler and c library, should I check for, and use if available, the builtins
for float, double and long double?

-- 
Steven King
sxking@uswest.net