This reverts commit 658f5e574f4025655da318fb0b688a394b6ff43f, which
was made without any justification, and prevents using crosstool-ng
even in decently recent distributions like Ubuntu 20.04 (which has
autoconf 2.69).
We have tested that ./bootstrap && ./configure --enable-local works
fine on Ubuntu 18.04, which uses autoconf 2.69 and automake 1.15, so
the existing requirements were enough.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>