https://bugs.gentoo.org/518440

avoid compiling tests when we aren't actually testing, especially since it
requires a compiler to execute which fails when cross-compiling

--- a/Makefile.am
+++ b/Makefile.am
@@ -135,7 +135,7 @@ t_generated_code2_test_generated_code2_SOURCES = \
 t_generated_code2_test_generated_code2_LDADD = \
 	protobuf-c/libprotobuf-c.la
 
-noinst_PROGRAMS += \
+check_PROGRAMS += \
 	t/generated-code2/cxx-generate-packed-data
 
 t_generated_code2_cxx_generate_packed_data_SOURCES = \
@@ -160,12 +160,6 @@ t/test-full.pb.cc t/test-full.pb.h: @PROTOC@ $(top_srcdir)/t/test-full.proto
 t/generated-code2/test-full-cxx-output.inc: t/generated-code2/cxx-generate-packed-data$(EXEEXT)
 	$(AM_V_GEN)$(top_builddir)/t/generated-code2/cxx-generate-packed-data$(EXEEXT) > $(top_builddir)/t/generated-code2/test-full-cxx-output.inc
 
-BUILT_SOURCES += \
-	t/test.pb-c.c t/test.pb-c.h \
-	t/test-full.pb-c.c t/test-full.pb-c.h \
-	t/test-full.pb.cc t/test-full.pb.h \
-	t/generated-code2/test-full-cxx-output.inc
-
 t_version_version_SOURCES = \
 	t/version/version.c
 t_version_version_LDADD = \