Allow make to fail if missing modules for test dependencies

e.g. make STRICT_TESTS=1

However we don't need this in .travis.yml as explained in the
comments.

.travis.yml

@@ -15,7 +15,9 @@ before_install:
 install:
   - autoreconf --install
   - eval `perl -V:siteprefix`
+  # Note: this will complain Test::Output isn't yet installed:
   - ./configure --prefix=$siteprefix && make
+  # but that's OK because we install it here:
   - make cpanm
 script:
   - make distcheck

configure.ac

@@ -102,6 +102,9 @@ EOF
 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 
 EOF
+    if test -n "$STRICT_TESTS"; then
+      exit 1
+    fi
   fi
 ]])
 AC_SUBST([PMDIR])