tools: Once again honour, but no longer advertise GIT_HTTP env var

In "build: add autoconf to replace custom checks in tools/check"
--enable-githttp was introduced.  But we missed this comment where it
was advertised.

Also, that commit had the effect of uncondtionally setting GIT_HTTP
from the configure variable.  But the env var has been advertised in
some places as the way to specify this behaviour, and overriding it is
just unfriendly.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

config/Tools.mk.in

@@ -40,8 +40,8 @@ XEN_TOOLS_RPATH     := @rpath@
 # Download GIT repositories via HTTP or GIT's own protocol?
 # GIT's protocol is faster and more robust, when it works at all (firewalls
 # may block it). We make it the default, but if your GIT repository downloads
-# fail or hang, please specify GIT_HTTP=y in your environment.
-GIT_HTTP            := @githttp@
+# fail or hang, please pass --enable-githttp to configure.
+GIT_HTTP            ?= @githttp@
 
 # Optional components
 XENSTAT_XENTOP      := @monitors@

config/Toplevel.mk.in

@@ -1,2 +1,2 @@
 SUBSYSTEMS               := @SUBSYSTEMS@
-GIT_HTTP                 := @githttp@
+GIT_HTTP                 ?= @githttp@