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.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Merge pull request #663 from luizluca/libvpx_issue662

libvpx: force to use gcc as linker