build: Set PERL5LIB globally for the test suite to the local modules directory

Regression introduced in commit 34b26c7db232b300991df5fb6f52707b381df28f.

Some functional tests are passing through makefile and exec barriers,
where the lib parameter for TAP::Harness is lost. Reintroduce the
explicit global setting of PERL5LIB so that it gets inherited for all
subprocesses.

Closes: #801329
Reported-by: Jérémy Bobbio <lunar@debian.org>
Stable-Candidate: 1.17.x

Makefile.am

@@ -156,7 +156,6 @@ test_data = \
 # while changing dir to source dir on out-of-tree builds.
 TEST_ENV_VARS = \
 	DPKG_DATADIR=$(abs_top_srcdir) \
-	PERL5LIB=$(abs_top_srcdir)/scripts:$(abs_top_srcdir)/dselect/methods \
 	$(nil)
 
 include $(top_srcdir)/check.am

check.am

@@ -31,6 +31,7 @@ check-local: $(test_data) $(test_programs) $(test_scripts)
 	  srcdir=$(srcdir) builddir=$(builddir) \
 	  CC=$(CC) \
 	  PERL_DL_NONLAZY=1 \
+	  PERL5LIB=$(abs_top_srcdir)/scripts:$(abs_top_srcdir)/dselect/methods \
 	  PERL5OPT=$(TEST_COVERAGE) \
 	  $(PERL) -MTAP::Harness -e $(TEST_RUNNER) \
 	    $(addprefix $(builddir)/,$(test_programs)) \

debian/changelog

@@ -4,6 +4,10 @@ dpkg (1.18.4) UNRELEASED; urgency=low
   * Switch dpkg-scansources and dpkg-scanpackages to use File::Find instead
     of find(1), as the former is more portable with more consistent behavior,
     and always canonicalizes the pathnames. Closes: #800649
+  * Build system:
+    - Set PERL5LIB globally for the test suite to the local modules directory,
+      to avoid using the system modules. Regression introduced in dpkg 1.17.8.
+      Reported by Jérémy Bobbio <lunar@debian.org>. Closes: #801329
   * Documentation:
     - Move description for “target architecture” from the dpkg-architecture(1)
       ‘-A’ option to the TERMS section. Closes: #799046