libgc: fix build under uClibc

fenv.h is used in a workaround for a glibc specific bug. However, uClibc lies
by defining __GLIBC__, but does not provide fenv.h in our default uClibc
configurations.

Fixes:
http://autobuild.buildroot.net/results/b2a/b2a1b1e374764855e701ea62fe9f1385ba77ac0d/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>