fix: modern clang compatibility (c++03, fgets, tolower, macOS ld)

[bqw371]

Summary

Three fixes required to build libretro-fceux with modern clang (Apple clang 15+ / LLVM 16+) on macOS arm64 or cross-compiling for arm64-Linux. All three are straightforward correctness fixes that silence hard errors (not just warnings).

1. src-fceux/cheat.cpp — fgets() pointer vs int comparison

fgets() returns char*, so comparing with >0 is a pointer-to-integer comparison, which is an error in modern clang (-Werror or by default in strict mode).

// before
while(fgets(linebuf,2048,fp)>0)
// after  
while(fgets(linebuf,2048,fp))

2. src-fceux/utils/xstring.cpp — conflicting tolower declaration

The file declares its own extern "C" int tolower(int) which conflicts with <cctype>'s tolower. Modern clang rejects conflicting linkage specifications.

// before: hand-rolled extern "C" declaration
// after: #include <ctype.h>

3. Makefile.libretro-fceux — C++03 mode + macOS linker flag

• Added -std=c++03 -Wno-narrowing -Wno-reserved-user-defined-literal to CXXFLAGS: required because the source uses C++03 idioms that are errors under C++11+ defaults in modern clang.
• Guard -Wl,-no-undefined inside ifneq ($(platform), osx): Apple ld does not support this flag.

Tested on

• macOS 14 (arm64), Apple clang 15 — platform=osx → libretro.dylib (494 KB)
• Cross-compile for arm64-Linux (aarch64-linux-gnu-g++) — platform=unix → libretro.so (~500 KB)
• Validated: SMB1 (HVC-SM) + HappyLee 4:57.31 fm2 (TASVideos #1715M) ran all 17,868 frames without crash via Python ctypes frontend.

Note on libretro-fceux/libretro.cp

The Makefile uses $(wildcard *.cpp) to collect source files, but the libretro shim is named libretro.cp (missing final p). A copy named libretro.cpp is needed alongside it. Not included in this patch as the right fix may be to update the Makefile glob to also match *.cp.