libserialport: Fix detection of OS when compiling for ucLinux

When compiling with a toolchain targetted at ucLinux, libserialport's
configure.ac would fail at detecting the target operating system.
As a result, the Linux-specific files were not compiled in that
particular case.

While this commit does not fix any autobuider failure for this package,
it fixes autobuilder failures for other packages, for instance:

http://autobuild.buildroot.net/results/69e/69e43bc171e554cf10f2ad526cebf5e0e524538a/



Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>