build: Provide gpgme.pc, generated by configure.
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Description

build: Provide gpgme.pc, generated by configure.

* configure.ac: Generate src/gpgme.pc.
* src/Makefile.am (pkgconfigdir, pkgconfig_DATA): New.
(EXTRA_DIST): Add gpgme.pc.in.
* src/gpgme.pc.in: New.
* src/gpgme-config.in: Use variables.

Some usages of gpgme-config is not compatible to pkg-config style;
The --glib option and --thread option which affect the output
by --cflags or --libs are not supported by gpgme.pc.

gpgme-config's embedding information for gpg-error and libassuan at
the build time of gpgme is considered inflexible than pkg-config
style. It is now handled by dependency of gpgme.pc (Requires field).

To use gpgme.pc, newer libgpg-error (>= 1.33) and libassuan (>= 2.5.3)
are required, which provide gpg-error.pc and libassuan.pc respectively.