libstdc++ provides the fenv.h found and for uclibc default toolchains
the bits/c++config.h has _GLIBCXX_HAVE_FENV_H undefined leading to
no import of the uclibc fenv.h
Change-Id: I53173b099f3d8791f527f1ccb60991de57cd19db
Filed: https://bugs.busybox.net/show_bug.cgi?id=4484
Reviewed-by: Harald Fernengel <harald.fernengel@nokia.com>