contrib/persistent-https: update ldflags syntax for Go 1.7+

Running `make all` in `contrib/persistent-https` results in a
failure on Go 1.7 and above.

Specifically, the error is:

    go build -o git-remote-persistent-https \
   -ldflags "-X main._BUILD_EMBED_LABEL 1468613136"
    # _/Users/parkr/github/git/contrib/persistent-https
    /usr/local/Cellar/go/1.7rc1/libexec/pkg/tool/darwin_amd64/link: -X
flag requires argument of the form importpath.name=value
    make: *** [git-remote-persistent-https] Error 2

This `name=value` syntax for the -X flag was introduced in Go v1.5
(released Aug 19, 2015):

 - release notes: https://golang.org/doc/go1.5#link
 - commit: golang/go@12795c0

In Go v1.7, support for the old syntax was removed:

 - release notes: https://tip.golang.org/doc/go1.7#compiler
 - commit: golang/go@51b624e

Add '=' between the symbol and its value for recent versions of Go,
while leaving it out for older ones.

Signed-off-by: Parker Moore <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: parkr

contrib/persistent-https/Makefile

@@ -25,8 +25,10 @@ git-remote-persistent-http: git-remote-persistent-https
 	ln -f -s git-remote-persistent-https git-remote-persistent-http
 
 git-remote-persistent-https:
+	case $$(go version) in \
+	"go version go"1.[0-5].*) EQ=" " ;; *) EQ="=" ;; esac && \
 	go build -o git-remote-persistent-https \
-		-ldflags "-X main._BUILD_EMBED_LABEL $(BUILD_LABEL)"
+		-ldflags "-X main._BUILD_EMBED_LABEL$${EQ}$(BUILD_LABEL)"
 
 clean:
 	rm -f git-remote-persistent-http* *.tar.gz