Added loadavg collector for solaris

It seems solaris prefers "sys/loadavg.h" over "stdlib.h" when
fetching the load average.

For Illumos based OSes it was required to include "sys/time.h" to
ensure that "hrtime_t" was defined.

https://www.illumos.org/issues/6002

It also required setting the ldflags "-fno-stack-protector -lssp" to
avoid undefined symbols when linking with gcc.

/opt/local/go/pkg/tool/solaris_amd64/link: running gcc failed: exit status 1
Undefined                       first referenced
 symbol                             in file
 __stack_chk_fail                    /tmp/go-link-138622936/000002.o
 __stack_chk_guard                   /tmp/go-link-138622936/000002.o

collector/loadavg_solaris.go

@@ -0,0 +1,40 @@
+// Copyright 2015 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build solaris
+// +build !noloadavg
+
+package collector
+
+import (
+	"errors"
+)
+
+/*
+// Define "__stack_chk_fail" and "__stack_chk_guard" symbols.
+#cgo LDFLAGS: -fno-stack-protector -lssp
+// Ensure "hrtime_t" is defined for sys/loadavg.h
+#include <sys/time.h>
+#include <sys/loadavg.h>
+*/
+import "C"
+
+func getLoad() ([]float64, error) {
+	var loadavg [3]C.double
+	samples := C.getloadavg(&loadavg[0], 3)
+	if samples > 0 {
+		return []float64{float64(loadavg[0]), float64(loadavg[1]), float64(loadavg[2])}, nil
+	} else {
+		return nil, errors.New("failed to get load average")
+	}
+}

collector/loadavg_unix.go

@@ -11,7 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build darwin dragonfly netbsd openbsd solaris
+// +build darwin dragonfly netbsd openbsd
 // +build !noloadavg
 
 package collector