perl: use mock i18n functions under NO_GETTEXT=Y

Change the logic of the i18n functions I added in 5e9637c629 (i18n:
add infrastructure for translating Git with gettext, 2011-11-18) to
use pass-through functions when NO_GETTEXT is defined.

This speeds up the compilation time of commands that use this library
when NO_GETTEXT=Y is in effect. Loading it and POSIX.pm is around 20ms
on my machine, whereas it takes 2ms to just instantiate perl itself.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1986,6 +1986,7 @@ ETC_GITCONFIG_SQ = $(subst ','\'',$(ETC_GITCONFIG))
 ETC_GITATTRIBUTES_SQ = $(subst ','\'',$(ETC_GITATTRIBUTES))
 
 DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
+NO_GETTEXT_SQ = $(subst ','\'',$(NO_GETTEXT))
 bindir_SQ = $(subst ','\'',$(bindir))
 bindir_relative_SQ = $(subst ','\'',$(bindir_relative))
 mandir_SQ = $(subst ','\'',$(mandir))
@@ -2276,6 +2277,7 @@ PERL_DEFINES += $(PERLLIB_EXTRA_SQ)
 PERL_DEFINES += $(perllibdir_SQ)
 PERL_DEFINES += $(RUNTIME_PREFIX)
 PERL_DEFINES += $(NO_PERL_CPAN_FALLBACKS)
+PERL_DEFINES += $(NO_GETTEXT)
 
 # Support Perl runtime prefix. In this mode, a different header is installed
 # into Perl scripts.
@@ -2680,6 +2682,7 @@ endif
 perl/build/lib/%.pm: perl/%.pm GIT-PERL-DEFINES
 	$(QUIET_GEN)mkdir -p $(dir $@) && \
 	sed -e 's|@@LOCALEDIR@@|$(perl_localedir_SQ)|g' \
+	    -e 's|@@NO_GETTEXT@@|$(NO_GETTEXT_SQ)|g' \
 	    -e 's|@@NO_PERL_CPAN_FALLBACKS@@|$(NO_PERL_CPAN_FALLBACKS_SQ)|g' \
 	< $< > $@
 

perl/Git/I18N.pm

@@ -16,9 +16,19 @@ BEGIN {
 our @EXPORT = qw(__ __n N__);
 our @EXPORT_OK = @EXPORT;
 
+# See Git::LoadCPAN's NO_PERL_CPAN_FALLBACKS_STR for a description of
+# this "'@@' [...] '@@'" pattern.
+use constant NO_GETTEXT_STR => '@@' . 'NO_GETTEXT' . '@@';
+use constant NO_GETTEXT => (
+	q[@@NO_GETTEXT@@] ne ''
+	and
+	q[@@NO_GETTEXT@@] ne NO_GETTEXT_STR
+);
+
 sub __bootstrap_locale_messages {
 	our $TEXTDOMAIN = 'git';
 	our $TEXTDOMAINDIR ||= $ENV{GIT_TEXTDOMAINDIR} || '@@LOCALEDIR@@';
+	die "NO_GETTEXT=" . NO_GETTEXT_STR if NO_GETTEXT;
 
 	require POSIX;
 	POSIX->import(qw(setlocale));