src/Makefile.am (libgpg_error_la_LDFLAGS): Always set -no-undefined.
(no_undefined): Drop Makefile variable.
The -no-undefined flag does tell libtool that the library to create does
not rely on undefined symbols by design, allowing libtool to create the
shared library even for platforms not supporting undefined symbols.
Beyond that, on platforms where supported, this allows libtool to add a
linker flag to complain about undefined symbols, to unhide them early
during development already.