gnupg2: fix build on non-wchar toolchains

The gnupg2 configure script checks whether <stdint.h> complies with
C99. When doing this, it expects a number of WCHAR_* definitions to be
present, which is not the case on non-wchar capable toolchains. The
gnupg2 configure script then concludes that <stdint.h> is not
C99-compliant and generates its own, which causes some build failures
related to intmax_t being not defined.

Since wchar is not actually used in gnupg2, this commit fixes this
problem by forcing gnupg2 to think that our <stdint.h> is
C99-compliant.

Fixes:

  http://autobuild.buildroot.org/results/40f/40fff3bc304e1a83524f28be8f6afc2e217281ad/



And lots of similar issues. Thanks a lot for Romain Naour for the
initial investigation and lots of discussion on IRC about this issue.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>