Using -ffast-math is causing linking problems like the one below in the
nightly builds with Clang 7 + GCC 6.2:

CMakeFiles/VecOps.dir/src/TVec.cxx.o: In function `exp':
 /.../include/c++/6.2.0/cmath:246: undefined reference to `__expf_finite'

This is because when linking with -ffast-math, the compiler is expecting
the symbol __expf_finite to be in libm.so, but it is not there:

$ nm -D /lib/libm-2.12.so | grep expf
00012010 W cexpf
00010aa0 W expf
000141d0 W frexpf
00014240 W ldexpf

The vectorized math functions were only introduced in glibc-2.22, so
on a machine with a more recent glibc, however, the symbols are there:

$ nm -D /lib/libm-2.26.so | grep expf
000000000004d5d0 W cexpf
000000000006bbe0 W cexpf128
000000000000f930 W expf
000000000006fb10 W expf128
0000000000050f20 T __expf128_finite
0000000000040de0 T __expf_finite
00000000000489b0 W frexpf
0000000000064080 W frexpf128
0000000000048a60 W ldexpf
00000000000641b0 W ldexpf128

More information: https://sourceware.org/glibc/wiki/libmvec