vlc: remove dependency on C library

vlc uses <spawn.h> which was not available in old uClibc
versions. However, since we are removing support for uClibc 0.9.33, we
can get rid of such dependency. In addition, <spawn.h> is provided by
musl, and therefore VLC can be enabled with this C library.

Consequently, this commit completely removes any C library dependency for
the vlc package.  The only special case that needs to be handled is the
Blackfin external toolchain from Analog Devices, which still uses an old
uClibc version that doesn't provide <spawn.h>, but as vlc uses fork() we add
a depends on BR2_USE_MMU (which covers the blackfin toolchain).

[Peter: add BR2_USE_MMU dependency]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>