Use the same perl6-m executable from configure time also at runtime

Fixes issues with precompilation failing due to a hard coded perl6 executable
name.

Makefile.PL

@@ -4,10 +4,11 @@ use ExtUtils::MakeMaker;
 # See lib/ExtUtils/MakeMaker.pm for details of how to influence
 # the contents of the Makefile that is written.
 
+my $perl6 = qx{perl6-m -e 'say \$*EXECUTABLE.absolute'};
+chomp $perl6;
 my $install_cmd = q{perl6-m -e 'say $*EXECUTABLE.dirname.Str.subst(rx{\/<-[\/]>+$}, "")'};
 my $install = qx{$install_cmd};
 chomp $install;
-say $install;
 
 my $moar_version_cmd = q{perl6-m -e 'say $*VM.version ge v2016.11.27.g.3171.dbba'};
 my $moar_version = qx{$moar_version_cmd};
@@ -35,6 +36,7 @@ WriteMakefile(
 );
 
 open my $header, '>', 'Perl6.h';
+print { $header } qq{#define PERL6_EXECUTABLE "$perl6"\n};
 print { $header } qq{#define PERL6_INSTALL_PATH "$install"\n};
 print { $header } $moar_version eq 'True'
     ? qq{#define mark_thread_blocked(tc) MVM_gc_mark_thread_blocked(tc)\n}

Perl6.xs

@@ -62,7 +62,7 @@ initialize()
 
         /* stash the rest of the raw command line args in the instance */
         instance->prog_name  = PERL6_INSTALL_PATH "/share/perl6/runtime/perl6.moarvm";
-        instance->exec_name  = "perl6";
+        instance->exec_name  = PERL6_EXECUTABLE;
         instance->raw_clargs = NULL;
 
         /* Map the compilation unit into memory and dissect it. */