Allow injecting additional arguments to test suite.
Useful to be able to run small subset of individual
test suite:

    make spec EXTRA_SPEC_FLAGS='-e parse_set_cookie'

--- a/Makefile
+++ b/Makefile
@@ -31 +31 @@ override FLAGS += $(if $(release),--release )$(if $(stats),--stats )$(if $(progr
-SPEC_FLAGS := $(if $(verbose),-v )$(if $(junit_output),--junit_output $(junit_output) )
+SPEC_FLAGS := $(if $(verbose),-v )$(if $(junit_output),--junit_output $(junit_output) )$(EXTRA_SPEC_FLAGS)