valgrind: bump to 3.7.0 and fix build with ccache

After bumping to 3.7.0, the valgrind-3.6.1-configure-support-linux-3.patch is
no longer necessary since it was added upstream. The
valgrind-3.7.0-compiler-check.patch patch is added to fix a compiler version
check issue (patch taken from the upstream Subversion repository). The
valgrind-3.7.0-fix-ccache-support.patch is added to fix the build process when
ccache is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>