package/Makefile.in: set TARGET_FCFLAGS variables

TARGET_FCFLAGS is already added to TARGET_CONFIGURE_OPTS, but was not
defined so far. This change fixes this.

Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Benjamin Kamath <bkamath@spaceflight.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>