package/gawk: clean the mpfr dependency

When available, mpfr provides support for BIGNUM, which allows gawk to
work on arbitrarily-large numbers (hence the name).

Forcibly disable mpfr if the mpfr package is not enabled. Let
./configure find it automatically when it is enabled.

To be noted, the host-gawk need not have BIGNUM to build the target
variant with support for BIGNUM.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>