Carry CFLAGS over to coverage build / make it work again

AM_CFLAGS are set by the recursive make, but it's CFLAGS
which we need to carry over.

src/Makefile.am

@@ -97,7 +97,7 @@ TESTS = @VMOD_TESTS@
 
 gcov: clean
 	$(AM_V_at)$(MAKE) $(AM_MAKEFLAGS) CC=gcc \
-		CFLAGS="${AM_CFLAGS} --coverage -fno-inline -g -O0" check
+		CFLAGS="${CFLAGS} --coverage -fno-inline -g -O0" check
 
 # Set QUIET=-q for non-verbose builds, otherwise set to empty.
 QUIET_0 = -q