pv: fix build issue with some toolchains

While 'pv' uses autoconf, it does not use automake for its
makefiles. It uses $(LD) $(LDFLAGS) to achieve partial linking, but
using 'ld' directly doesn't work well with some toolchain
configuration, as the ld default emulation may not necessarily be the
correct one. By passing custom values for LD and LDFLAGS, we ensure
that 'gcc' is used to do these partial linking steps.

Fixes the following build failures (x86-64 and mips64) :

  http://autobuild.buildroot.org/results/9ac/9aca276a220ad1460e5b32bd042dd4c93649baf3/build-end.log
  http://autobuild.buildroot.org/results/910/91044c01dcc3c4150aa6e74f5964fff27b9dac79/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>