In the Makefile, `FFLAGS` was being set to the value of `CFLAGS` which no
longer works when the flags are being tuned for the Clang compiler. `FCFLAGS`
is now used instead as it contains a copy of `CFLAGS` that has been edited for
use by GFortran.
FixesHomebrew/homebrew#11119.
qrupdate is a Fortran library for fast updates of QR and Cholesky
decompositions. It is used by Octave.
Signed-off-by: Adam Vandenberg <flangy@gmail.com>