[clisp-list] fenv.h support

[Will N. <wi...@mi...>]

CLISP relies on the fpu_control.h header to set the FPU behaviour on Linux. 
Unfortunately, fpu_control.h is unportable in many ways. The general anwser 
seems to be to use fenv.h, a standard header in C99. This patch will enable 
fenv.h support in preference to fpu_control.h if it is available. I have 
tested it and it *seems* to work. I've tried dividing by zero etc. in the 
interpreter and everything seems to function normally on my i386 Linux 
machine. It would be best if this patch got some testing before being 
considered for inclusion.

Thanks,