Now with meson build system libsecret FTBFS without valgrind installed.
Two issues: (1) egg-testing.c depends on valgrind/valgrind.h unnecessarily;
(2) we forgot to add "build" directory as a header directory for meson.
This commit resolves both of them.
Some distributions patch libgcrypt to provide a pkg-config file, and
disable libgcrypt-config. One of these distributions is Yocto, which we
use as the basis for the GNOME Continuous build environment.
We should check if there's a pkg-config file available for gcrypt, and
use it, falling back to libgcrypt-config if the pkg-config file is not
found.
