[PATCH] tests: honour external CFLAGS in test_lib.sh

The test harness hardcoded CFLAGS="-I$TEST_DIR/includes", overwriting
any externally-supplied CFLAGS. That works on a Hurd development box
(or any host with Mach userland headers installed under standard
search paths) but breaks `make check` on every other host: the
generated stubs `#include <mach/boolean.h>` etc., and the test
includes/ tree only ships `mach/mig_support.h` — the rest were
implicitly expected to come from the system's installed Mach
headers.

Append rather than overwrite, so cross-build users can supply their
target's installed Mach headers via the standard CFLAGS env var:

    make check CFLAGS="-I/path/to/dist/include"

Test-suite stubs still take precedence (they come first on the
include path), so any test relying on a stub definition is
unaffected.

Verified by running `make check` on macOS aarch64 with a freshly
installed gnumach header tree — all 7 tests in tests/good/ now
pass; bad/ and generate-only/ behaviour unchanged.
---
 tests/test_lib.sh | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tests/test_lib.sh b/tests/test_lib.sh
index 60241d9..43ef738 100644
--- a/tests/test_lib.sh
+++ b/tests/test_lib.sh
@@ -20,7 +20,13 @@
 
 MIGCOM="$BUILDDIR/migcom"
 TEST_DIR="$SRCDIR/tests"
-CFLAGS="-I$TEST_DIR/includes"
+
+# Honour an externally-supplied CFLAGS so that cross-build / non-Hurd
+# hosts can point the test harness at their installed Mach userland
+# headers (e.g. `make check CFLAGS=-I/path/to/mach/include`). The
+# test-suite's own stubs come first so test-controlled definitions
+# always win, with external paths searched as a fallback.
+CFLAGS="-I$TEST_DIR/includes ${CFLAGS:-}"
 
 failure () {
   msg="$1"
-- 
2.54.0