lib/lnag-utils.h: Fix compile error in non-gnu compatibility code

We need the lnag_clz() function, which gcc is kind enough to provide
a builtin for but Solaris cc (and a bunch of others, I'm sure) needs
help with.

Reported-by: Gaspar Carson <Carson.Gaspar@gs.com>
Signed-off-by: Andreas Ericsson <ae@op5.se>
lib/Makefile: Stop hardcoding gcc as our compiler

We get @CC@ substituted for us, which includes the users choice
of compiler.

This means also removing -Wall, which not all compilers understand.

Reported-by: Gaspar Carson <Carson.Gaspar@gs.com>
Signed-off-by: Andreas Ericsson <ae@op5.se>