gsl: fix build on !i386 uClibc

uClibc pretends to implement <fenv.h> as it installs the header, but
in practice, it only implements the functions for i386. This makes gsl
unhappy as it detects fenv.h, but then cannot use the fenv functions.

Fixes:

  http://autobuild.buildroot.org/results/732cc07faeca2a9098dc5106e8f654eb1323451a/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>