Makefile: support overriding CGO_ENABLED

We generally want CGO_ENABLED=0, but cgo is required for odroid.
This change makes the following work:

make manager CGO_ENABLED=1 GOTAGS=odroid
diff --git a/Makefile b/Makefile
index 6cd7a51..fa9669e 100644
--- a/Makefile
+++ b/Makefile
@@ -40,7 +40,8 @@
 HOSTGO := go
 # By default, build all Go binaries as static. We don't need cgo and it is
 # known to cause problems at least on Android emulator.
-export CGO_ENABLED=0
+CGO_ENABLED ?= 0
+export CGO_ENABLED
 TARGETGOOS := $(TARGETOS)
 TARGETGOARCH := $(TARGETVMARCH)