libc/uClibc: add option to enable fenv

Support for fenv.h is a little bit more tricky that enabling it only
for x86-32 is not right.

Add an option for the user to choose whther to install fenv.h or not.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"