Define _DEFAULT_SOURCE when compiling the port driver

With -std=c17, musl's string.h hides the POSIX.1-2008 strnlen
declaration, and GCC 14 (Alpine 3.24) turns the resulting implicit
declaration into a hard error.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: maennchen

c_src/Makefile

@@ -94,6 +94,9 @@ ifneq ($(CROSSCOMPILER),)
 	CC = $(CROSSCOMPILER)gcc
 endif
 
+# Expose POSIX.1-2008 declarations (e.g. strnlen) despite -std=c17.
+CPPFLAGS += -D_DEFAULT_SOURCE
+
 CFLAGS += -I"$(ERTS_INCLUDE_DIR)" -I"$(ERL_INTERFACE_INCLUDE_DIR)" -I"$(LIBSODIUM_INCLUDE_DIR)"
 CXXFLAGS += -I"$(ERTS_INCLUDE_DIR)" -I"$(ERL_INTERFACE_INCLUDE_DIR)" -I"$(LIBSODIUM_INCLUDE_DIR)"