Add -mlzcnt on GCC compilations.

Closes: https://bugs.gentoo.org/739776
Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>

--- src/Makefile.am
+++ src/Makefile.am
@@ -224,7 +224,7 @@
 if HAVE_X86
 
 if HAVE_AVX2_GCC
-libavx2_la_CFLAGS = -mavx2 -mbmi -mabm -mbmi2
+libavx2_la_CFLAGS = -mavx2 -mbmi -mabm -mlzcnt -mbmi2
 endif
 if HAVE_AVX2_CLANG
 libavx2_la_CFLAGS = -mavx2 -mbmi -mpopcnt -mlzcnt -mbmi2