Fix open_memstream/fmemopen feature detection with GCC >= 14 (#544)

GCC 14 has enabled various warnings as errors by default, e.g.
-Wimplicit-function-declaration. This causes the current feature
detection code for `open_memstream(3)` and `fmemopen(3)` to fail
with GCC 14.

This commit restores compatibility with GCC 14 in this regard.

Note that it may also be beneficial to pass a feature test macro
such as -D_POSIX_C_SOURCE. See the feature test macro requirements
for open_memstream(3)` and `fmemopen(3)`.

Makefile.config

@@ -91,8 +91,9 @@ DESTDIR   ?=
 
 # Automatically detect platform-specific flags, instead of using autoconf
 #CYC_PLATFORM_HAS_MEMSTREAM ?= 1
-CYC_PLATFORM_HAS_MEMSTREAM := $(shell echo "main(){char *buf; int len; open_memstream(&buf, &len);}" | $(CC) -xc - >/dev/null 2>/dev/null && echo 1 || echo 0)
-CYC_PLATFORM_HAS_FMEMOPEN := $(shell echo "main(){char *buf; fmemopen(&buf, 0, \"r\");}" | $(CC) -xc - >/dev/null 2>/dev/null && echo 1 || echo 0)
+HASH := \# # Needed for compatibility with GNU Make < 4.3 <https://lists.gnu.org/archive/html/info-gnu/2020-01/msg00004.html>
+CYC_PLATFORM_HAS_MEMSTREAM := $(shell printf "$(HASH)include <stdio.h>\n%s\n" "int main(void){char *buf; size_t len; open_memstream(&buf, &len); return 0;}" | $(CC) -xc - >/dev/null 2>/dev/null && echo 1 || echo 0)
+CYC_PLATFORM_HAS_FMEMOPEN := $(shell printf "$(HASH)include <stdio.h>\n%s\n" "int main(void){char *buf; fmemopen(&buf, 0, \"r\"); return 0;}" | $(CC) -xc - >/dev/null 2>/dev/null && echo 1 || echo 0)
 
 # code from chibi's makefile to detect platform
 ifndef PLATFORM