fix(build): Fix build configuration for --with-system-libtexpdf

Makefile.am

@@ -6,7 +6,7 @@ ACLOCAL_AMFLAGS = -I build-aux
 .DELETE_ON_ERROR:
 
 SUBDIRS = justenough
-if SYSTEM_LIBTEXPDF
+if !SYSTEM_LIBTEXPDF
 SUBDIRS += libtexpdf
 endif
 
@@ -151,7 +151,9 @@ $(_BUILT_SUBDIRS): $(_SUBDIR_TELLS)
 	touch $@
 
 $(_SUBDIR_TELLS):
+if !SYSTEM_LIBTEXPDF
 	$(MAKE) $(AM_MAKEFLAGS) -C libtexpdf all
+endif
 	$(MAKE) $(AM_MAKEFLAGS) -C justenough all
 
 patterndeps = $(_FORCED) $(_TEST_DEPS) $(_DOCS_DEPS) | $(bin_PROGRAMS) $(DEPDIRS) $(LUAMODLOCK) $(_BUILT_SUBDIRS)