Add a verbose mode to builds

Using the VERBOSE environment variable, tests and binary compilation are
ran in verbose mode (using -v), but by default there are more quiet O:).

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

Makefile

@@ -4,6 +4,7 @@ TRAEFIK_ENVS := \
 	-e OS_ARCH_ARG \
 	-e OS_PLATFORM_ARG \
 	-e TESTFLAGS \
+	-e VERBOSE \
 	-e VERSION
 
 SRCS = $(shell git ls-files '*.go' | grep -v '^external/')