build,qt: Build Qt with -fPIC if required or requested
* configure.ac: Add option to enable building the Qt 6 binding with -fPIC. Fix typo and mention default for --enable-no-direct-extern-access option. * m4/qt6.m4: Add -fPIC to GPGME_QT6_CFLAGS if requested or if Qt 6 was built with reduce_relocations.
- GnuPG-bug-id: T6781