configure.ac: disable maintainer mode by default
Tweak and cleanup AC_FIND_GAP macro

The only user visible change is that now instead of
  "GAP was compiled without GMP"
we print the less ambiguous message
  "GAP was compiled without its own GMP"
Version 4.4.3
Fix IO_Write to handle long input that is not in IsStringRep

The C function IO_write expects the string input to be in IsStringRep,
but a bug in the GAP function IO_Write passed parts of long Strings
directly to IO_write, which lead to Strings not being writable when
the write buffer overflowed.

This patch makes IO_Write copy all input into the write buffer, if the
input is not in IsStringRep, before calling IO_write on the write buffer.