package/boost: Disable fenv support with uClibc

The boost build system does not recognize the fact that fenv.h is an
optional module in uClibc and tries to use it even if UCLIBC_HAS_FENV
is disabled. This patch disables fenv support completely when compiling
with a uClibc-based toolchain.

Fixes
http://autobuild.buildroot.net/results/160/160e1b98b204148ecf128144826554b6c523931b/


and many others

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>