install: Properly install manpages in mandir/man1 which also fixes Bug#1064.
build: Hide glibc workarounds in PLATFORM_LINUX because it does only make sense on Linux.
build: Fix clang compiler warning
freebsd: Assume all devices on FreeBSD are USB 2.0 per default because
FreeBSD does not have sysfs to read the device speed and type properly.
The good thing is that there are no PCI devices that support V4L right
now so all supported devices are from webcamd which only supports USB
devices so that default is always correct.