compiler: support storing temporary files under TMPDIR/v/

Fix for filepath.join not \0 terminating its result

vlib/compiler/main.v

@@ -799,6 +799,7 @@ pub fn new_v(args[]string) &V {
 	vgen_buf.writeln('module vgen\nimport strings')
 
 	joined_args := args.join(' ')
+		
 	target_os := get_arg(joined_args, 'os', '')
 	comptime_define := get_arg(joined_args, 'd', '')
 	//println('comptimedefine=$comptime_define')
@@ -930,8 +931,8 @@ pub fn new_v(args[]string) &V {
 		println('Go to https://vlang.io to install V.')
 		exit(1)
 	}
-	// println('out_name:$out_name')
-	mut out_name_c := os.realpath('${out_name}.tmp.c')
+
+	mut out_name_c := get_vtmp_filename( out_name, '.tmp.c')
 
 	cflags := get_cmdline_cflags(args)
 
@@ -978,7 +979,7 @@ pub fn new_v(args[]string) &V {
 		println('C compiler=$pref.ccompiler')
 	}
 	if pref.is_so {
-		out_name_c = out_name.all_after(os.path_separator) + '_shared_lib.c'
+		out_name_c = get_vtmp_filename( out_name, '.tmp.so.c')
 	}
 	$if !linux {
 		if pref.is_bare && !out_name.ends_with('.c') {

vlib/compiler/vhelp.v

@@ -28,6 +28,9 @@ pub const (
    You can set it like this: `export VFLAGS="-cc clang -debug"` on *nix,
    `set VFLAGS=-cc msvc` on Windows.
 
+   V respects the TMPDIR environment variable, and will put .tmp.c files in TMPDIR/v/ .
+   If you have not set it, a suitable platform specific folder (like /tmp) will be used.
+
 Options/commands:
   -h, help          Display this information.
   -o <file>         Write output to <file>.

vlib/compiler/vtmp.v

@@ -0,0 +1,21 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
+module compiler
+
+import os
+import filepath
+
+pub fn get_vtmp_folder() string {
+	vtmp := filepath.join(os.tmpdir(),'v')
+	if !os.dir_exists( vtmp ) {
+		os.mkdir(vtmp)
+	}
+	return vtmp
+}
+
+pub fn get_vtmp_filename(base_file_name string, postfix string) string {
+	vtmp := get_vtmp_folder()
+	return os.realpath( filepath.join(vtmp, os.filename( base_file_name ) + postfix) )
+}