macos: somehow allow building with a target below 11.0

While building for macOS older than 11 in Apple Silicon makes no
sense, some build systems lack the flexibility to set a target per
architecture while aiming to support multi architecture binaries.

Allow an option in those cases by using the slower runtime checks
if the toolchain allows it.

Fixes: PCRE2Project/pcre2#109

Author: carenas

sljit_src/sljitExecAllocator.c

@@ -152,6 +152,9 @@ static SLJIT_INLINE void apple_update_wx_flags(sljit_s32 enable_exec)
 {
 #if MAC_OS_X_VERSION_MIN_REQUIRED >= 110000
 	pthread_jit_write_protect_np(enable_exec);
+#elif defined(__clang__)
+	if (__builtin_available(macOS 11.0, *))
+		pthread_jit_write_protect_np(enable_exec);
 #else
 #error "Must target Big Sur or newer"
 #endif /* BigSur */