build: Explicitly look for gpgmepp

build: Explicitly look for gpgmepp

* configure.ac: Require gpg-error 1.47, gpgme 1.24.0, and gpgmepp
1.24.0. Use AM_PATH_GPGMEPP to look for gpgmepp. Set GPGMEPP_CXXFLAGS to
value of GPGMEPP_CFLAGS set by AM_PATH_GPGMEPP. Change error message
shown if gpgmepp wasn't found.
* m4/Makefile.am (EXTRA_DIST): Add gpgmepp.m4.
* m4/gpgmepp.m4: New.
* src/Makefile.am (libgpgmepp.a): Use "gpgrt-config gpgmepp --prefix"
instead of "gpgme-config --prefix".
--

This prepares gpgol for the separation of gpgmepp from gpgme. Since
version 1.24.0 gpgme/gpgmepp provides a pkg-config file for gpgmepp
which allows us to look for gpgmepp with gpgrt-config. gpgme 1.24.0
requires gpg-error 1.47 so that we can safely require the same version.

GnuPG-bug-id: 7262