config: allow configure to continue without pkg-config installed

libusb only uses pkg-config macros in configure to check for umockdev. Since
this is only required for testing it makes sense to protect the usage of these
macros to only be used when pkg-config is installed. This will be the case for
maintainers and testers.

Signed-off-by: Nathan Hjelm <hjelmn@google.com>
2 files changed