Fix the LIBDCDEVEL=1 build when libdivecomputer was shadow-built

When that happens, the include/version.h file is in the shadow build
dir, not in the sources.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Search pkg-config before {xml2,xslt}-config

Cross-compilation tools provide the right .pc files and a pkg-config to
match, but often don't provide the scripts to match. Even if they did,
you'd have to change PATH to find them. So give pkg-config precedence.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>