run.bash: skip -static cgo test if -static linking isn't possible.

Some system doesn't have libc.a available.

While we're at here, also export GOROOT in run.bash, so that
one doesn't need to set GOROOT to run run.bash.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/99870043

src/run.bash

@@ -6,6 +6,7 @@
 set -e
 
 eval $(go env)
+export GOROOT   # the api test requires GOROOT to be set.
 
 unset CDPATH	# in case user has it set
 unset GOPATH    # we disallow local import for non-local packages, if $GOROOT happens
@@ -140,7 +141,11 @@ dragonfly-386 | dragonfly-amd64 | freebsd-386 | freebsd-amd64 | freebsd-arm | li
 	                # static linking on FreeBSD/ARM with clang. (cgo depends on
 			# -fPIC fundamentally.)
 	*)
-		go test -ldflags '-linkmode=external -extldflags "-static -pthread"' ../testtls || exit 1
+		if ! $CC -xc -o /dev/null -static - 2>/dev/null <<<'int main() {}' ; then
+			echo "No support for static linking found (lacks libc.a?), skip cgo static linking test."
+		else
+			go test -ldflags '-linkmode=external -extldflags "-static -pthread"' ../testtls || exit 1
+		fi
 		;;
 	esac
 	;;