compiler: first step to require explicit definition of imported C fns

compiler/parser.v

@@ -1582,6 +1582,10 @@ fn (p mut Parser) name_expr() string {
 		cfn := p.table.find_fn(name) or {
 			// Not Found? Return 'void*'
 			//return 'cvoid' //'void*'
+			if false {
+			p.warn('\ndefine imported C function with ' +
+				'`fn C.$name([args]) [return_type]`\n')
+			}
 			return 'void*'
 		}
 		return cfn.typ

vlib/builtin/array.v

@@ -33,6 +33,7 @@ pub fn _make(len, cap, elm_size int) array {
 	return new_array(len, cap, elm_size)
 }
 
+
 // Private function, used by V (`nums := [1, 2, 3]`)
 fn new_array_from_c_array(len, cap, elm_size int, c_array voidptr) array {
 	arr := array {

vlib/builtin/builtin.v

@@ -4,6 +4,12 @@
 
 module builtin
 
+fn C.memcpy(byteptr, byteptr, int)
+fn C.memmove(byteptr, byteptr, int)
+//fn C.malloc(int) byteptr
+fn C.realloc(byteptr, int) byteptr
+
+
 pub fn exit(code int) {
 	C.exit(code)
 }

vlib/math/math.v

@@ -4,8 +4,6 @@
 
 module math
 
-#include <math.h>
-
 // NOTE
 // When adding a new function, please make sure it's in the right place.
 // All functions are sorted alphabetically.
@@ -51,6 +49,8 @@ pub fn abs(a f64) f64 {
 	return a
 }
 
+fn C.acos(a f64) f64
+
 // acos calculates inverse cosine (arccosine).
 pub fn acos(a f64) f64 {
 	return C.acos(a)