Any libs needed when building static binaries must be listed in the
'Requires.private' field of the installed pkg-config file libserialport.pc.
Currently, this is libudev (but only when building on/for Linux).
The default behaviour of PKG_CHECK_MODULES is to abort with an error
anyway, but it will also print a more usable error message and hints
for the user about how to use PKG_CONFIG_PATH and so on.
