cc/gcc: add option to enable/disable decimal floats

Decimal floats need support form the C library, namely support
for fenv, which is missing in uClibc for any architecture but
x86/32.

Add an option (a choice) to enable or disable decimal floats.

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"