Allow niecza to be used from outside the source tree

It's not installable, but you can symlink it into $PATH and it will be able to
find the standard libraries and support code.

niecza_eval

@@ -3,7 +3,12 @@ use warnings;
 use strict;
 use 5.010;
 
-use lib 'src';
+BEGIN {
+    use FindBin;
+    use File::Spec;
+    unshift @INC, File::Spec->catdir($FindBin::RealBin, "src");
+}
+
 use CompilerDriver ':all';
 use Getopt::Long;
 use autodie ':all';

src/CompilerDriver.pm

@@ -13,9 +13,10 @@ use autodie ':all';
 
 open ::NIECZA_OUT, ">&", \*STDOUT;
 
-use lib 'STD_checkout';
 BEGIN {
     use File::Spec;
+    unshift @INC, File::Spec->catdir(
+        dirname(dirname($INC{'CompilerDriver.pm'})), 'STD_checkout');
     $CursorBase::SET_PERL6LIB = [ File::Spec->curdir ];
 }