valgrind: fix musl compile

valgrind: fix musl compile

Add musl libc detection (based on suggestions by Romain Naour and
Arnout Vandecappelle) and add some tweaks for musl support (based
on OpenWRT patch [1]).

Fixes ([2]):
  checking the glibc version... unsupported version
  configure: error: Valgrind requires glibc version 2.2 or later

Patch suggested upstream (see [3]).

[1] https://dev.openwrt.org/browser/trunk/package/devel/valgrind/patches/200-musl_fix.patch?rev=46302
[2] http://autobuild.buildroot.org/results/7b0/7b048ba58918f0a08498c61327fcf35a85a84837
[3] https://bugs.kde.org/show_bug.cgi?id=359202

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>