make $if work with includes: skip the body if the target is different

vlib/runtime/runtime.v

@@ -1 +1,14 @@
 module runtime
+
+$if linux {
+	#include <sys/sysinfo.h>
+	fn C.get_nprocs() int
+}
+
+
+pub fn nr_cpus() int {
+	$if linux {
+		return C.get_nprocs()
+	}
+	return 0
+}

vlib/runtime/runtime_mac.v

@@ -1,5 +1,2 @@
 module runtime
 
-pub fn nr_cpus() int {
-	return 0
-}

vlib/runtime/runtime_test.v

@@ -2,8 +2,8 @@ import runtime
 
 fn test_nr_cpus() {
 	$if linux {
-	nr_cpus := runtime.nr_cpus()
-	println(nr_cpus)
-	assert nr_cpus > 0
+		nr_cpus := runtime.nr_cpus()
+		println(nr_cpus)
+		assert nr_cpus > 0
 	}
 }	

vlib/runtime/runtime_win.v

@@ -1,5 +1,2 @@
 module runtime
 
-pub fn nr_cpus() int {
-	return 0
-}