perl.c: Improve buildcustomize error message

buildcustomize.pl was being loaded with generated code that died if
‘lib/buildcustomize.pl’ existed but do-file failed.  $@ was used as
the error message.

If the file does not exist (do-file searching @INC), then $@ is empty
and the error is in $!.  Before 9054c817, this is what we would get:

$ ./miniperl -T -Ilib -e0
Died.
BEGIN failed--compilation aborted.

I fixed that in 9054c817 so there would be no error (by adding an ini-
tial ‘./’ to the path.)

That message was rather baffling.  It would have been easier to track
down the cause if I had seen something like this:

$ ./miniperl -T -Ilib -e0
lib/buildcustomize.pl: No such file or directory.
BEGIN failed--compilation aborted.

This commit changes the generated code to output $! like that if $@ is
empty, in case a similar bug occurs again.

1 files changed, 3 insertions, 4 deletions

diff --git a/perl.c b/perl.c
index 4af534877b..b61e2ffd07 100644
--- a/perl.c
+++ b/perl.c
@@ -2070,10 +2070,9 @@ S_parse_body(pTHX_ char **env, XSINIT_t xsinit)
                it should be reported immediately as a build failure.  */
 	    (void)Perl_av_create_and_unshift_one(aTHX_ &PL_preambleav,
 						 Perl_newSVpvf(aTHX_
-				"BEGIN { do {local $!; -f q%c./%"SVf
-				"/buildcustomize.pl%c} and do q%c./%"SVf
-				"/buildcustomize.pl%c || die $@ }",
-							       0, SVfARG(*inc0), 0,
+		"BEGIN { my $f = q%c./%"SVf"/buildcustomize.pl%c; "
+			"do {local $!; -f $f }"
+			" and do $f || die $@ || qq '$f: $!' }",
 							       0, SVfARG(*inc0), 0));
 	}
 #  else