Fix test suite with GCC 15.

It's now smart enough to optimise out the calls to malloc, which is a bit of a
problem when the test suite is trying to test it!

From: https://github.com/jemalloc/jemalloc/issues/2823

--- 5.3.0/Makefile.in	2022-05-06 20:23:54.000000000 +0100
+++ 5.3.0/Makefile.in	2025-07-04 04:15:18.551615570 +0100
@@ -654,6 +654,7 @@
 endif
 
 tests_unit: $(TESTS_UNIT:$(srcroot)%.c=$(objroot)%$(EXE))
+tests_integration: CFLAGS += -fno-builtin
 tests_integration: $(TESTS_INTEGRATION:$(srcroot)%.c=$(objroot)%$(EXE)) $(TESTS_INTEGRATION_CPP:$(srcroot)%.cpp=$(objroot)%$(EXE))
 tests_analyze: $(TESTS_ANALYZE:$(srcroot)%.c=$(objroot)%$(EXE))
 tests_stress: $(TESTS_STRESS:$(srcroot)%.c=$(objroot)%$(EXE))