libvpx: force to use gcc as linker

libvpx excepts gcc to be used as linker. However, it respects
what is defined in LD. The problem is that LD is defined by
OpenWRT as *-ld.

Forcing the LD env for configure and make solves the problem.
Also, the patch that modified ld call to match what *-ld provides
is not needed anymore.

Fixes #662 

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>