build: Omit -L<libdir> for standard paths on some 64-bit systems

* configure.ac (GPG_ERROR_CONFIG_LIBS): Handle the case $libdir =
'${exec_prefix}/lib64'.

This ensures that on systems using /usr/lib64 the Libs entry of
gpg-error.pc doesn't get prefixed with -L/usr/lib64 which helps prevent
library search path problems when linking something that depends on
libgpg-error.

• GnuPG-bug-id: T6136