gpgme: Fix issue building for the target

gpgme: Fix issue building for the target

gpgme failed when configuring since you can only configure for one python
system at a time (via the inherits).  So we need to have a PACKAGECONFIG
that defines which one [or neither] you want to use.

The prior pkgconfig patch introduced the usage of the variable PKG_CONFIG,
which is not defined anywhere.  Define this.

When building the python module, we can not call gpg-error-config, so we
need to find an alternative way of finding the information the setup.py.in
requires.  (In this case, it's easy to just use the environment
STAGING_INCDIR.)

(From OE-Core rev: 839d6f124c2761194c868cf5597e1aa96571e1ca)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>