build: Add kludge for "make distcheck" in a release build.

* configure.ac: New option --enable-gnupg-builddir-envvar.
(ENABLE_GNUPG_BUILDDIR_ENVVAR): New ac_define.
* common/homedir.c (gnupg_set_builddir_from_env): Consider
ENABLE_GNUPG_BUILDDIR_ENVVAR.
* Makefile.am (DISTCHECK_CONFIGURE_FLAGS): Rename to ...
(AM_DISTCHECK_CONFIGURE_FLAGS): this to be future proof.  Add option
--enable-gnupg-builddir-envvar.

Our regression test suite makes use of the envvar GNUPG_BUILDDIR. Now
the code in gnupg for evaluating this envvar is only included in a
development version (that is one with a "-betaNNN" suffix). For a
real release the envvar is not considered. However during a "make
distcheck" a "make check" is done for the build directory. Without
defining that envar we would try to run binaries in the install
directory ("_inst" sub-directory) which are not yet installed at that
time.

• Signed-off-by: Werner Koch <wk@gnupg.org>