tests: prevent inclusion of time.h in baremetal compiles

baremetal compiles should not include time.h, as MBEDTLS_HAVE_TIME is
undefined. To test this, provide an overriding include directory that
has a time.h which throws a meaningful error if included.

Signed-off-by: Daniel Axtens <dja@axtens.net>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 0c2d9dd..7b25195 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -1435,9 +1435,15 @@
 component_build_crypto_baremetal () {
   msg "build: make, crypto only, baremetal config"
   scripts/config.py crypto_baremetal
-  make CFLAGS='-O1 -Werror'
+  make CFLAGS="-O1 -Werror -I$(pwd)/tests/include/baremetal-override/"
   are_empty_libraries library/libmbedx509.* library/libmbedtls.*
 }
+support_build_crypto_baremetal () {
+    # Older Glibc versions include time.h from other headers such as stdlib.h,
+    # which makes the no-time.h-in-baremetal check fail. Ubuntu 16.04 has this
+    # problem, Ubuntu 18.04 is ok.
+    ! grep -q -F time.h /usr/include/x86_64-linux-gnu/sys/types.h
+}
 
 component_test_depends_curves () {
     msg "test/build: curves.pl (gcc)" # ~ 4 min