Rebuild error2.h on Makefile changes.

Rebuild error2.h on Makefile changes.

Re-running configure without a subsequent make clean currently fails if
the set of objects has changed since the last run. This happens for
example if a different cryptolib was requested with --enable-cryptolib,
or if different libraries were detected by configure.

The problem is that error2.h is not updated according to the new
set of objects. Letting error2.h depend on Makefile should fix it,
because configure creates Makefile, so error2.h will be recreated
after reconfiguring.