compiler: support storing temporary files under TMPDIR/v/

Fix for filepath.join not \0 terminating its result

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>.