build: When no gpg-error-config, not install libassuan-config.

* configure.ac (USE_GPGRT_CONFIG): New.
* src/Makefile.am [USE_GPGRT_CONFIG]: Conditionalize the install
of libassuan-config.

When system will migrate use of gpgrt-config and removal of
gpg-error-config, libassuan-config will not be installed (but use
libassuan.pc by gpgrt-config).

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>