tests: avoid two spurious failures on FreeBSD

tests: avoid two spurious failures on FreeBSD

Due to the FreeBSD make bug that causes of the well-known automake
bug#7884, two of our bison-requiring tests were failing on FreeBSD.
To avoid this, simply skip the part of these tests affected by that
bug.

* t/yacc-bison-skeleton.sh: Here, using the 'yl_distcheck' function.
* t/yacc-bison-skeleton-cxx.sh: Likewise.
* t/yacc-nodist.sh: While at it, use the 'yl_distcheck' here
as well, instead of running "$MAKE distcheck" only when $MAKE
is GNU make.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>