Add flags to support OpenBSD

This commit includes OpenBSD in the platform checks used to check for
size_t, uint64_t, unsigned long and unsigned long long all being 64
bits in length.

Addtionally, the checks for some `avx513*` flags are removed, as
`avx512fp16` yield an error: "invalid cpu feature string for builtin"
when compiling with clang.

This commit aims to address #156

lib/x86simdsort.cpp

@@ -10,7 +10,8 @@ static int check_cpu_feature_support(std::string_view cpufeature)
     const char *disable_avx512 = std::getenv("XSS_DISABLE_AVX512");
 
     if ((cpufeature == "avx512_spr") && (!disable_avx512))
-#if defined(__FLT16_MAX__) && !defined(__INTEL_LLVM_COMPILER)
+#if defined(__FLT16_MAX__) && !defined(__INTEL_LLVM_COMPILER) \
+        && __clang_major__ >= 18
         return __builtin_cpu_supports("avx512f")
                 && __builtin_cpu_supports("avx512fp16")
                 && __builtin_cpu_supports("avx512vbmi2");

src/avx2-64bit-qsort.hpp

@@ -406,10 +406,10 @@ struct avx2_vector<uint64_t> {
 };
 
 /*
- * workaround on 64-bit macOS which defines size_t as unsigned long and defines
- * uint64_t as unsigned long long, both of which are 8 bytes
+ * workaround on 64-bit macOS and OpenBSD which both define size_t as unsigned
+ * long and define uint64_t as unsigned long long, both of which are 8 bytes
  */
-#if defined(__APPLE__) && defined(__x86_64__)
+#if (defined(__APPLE__) || defined(__OpenBSD__)) && defined(__x86_64__)
 static_assert(sizeof(size_t) == sizeof(uint64_t),
               "Size of size_t and uint64_t are not the same");
 template <>

src/avx512-64bit-common.h

@@ -960,10 +960,10 @@ struct zmm_vector<uint64_t> {
 };
 
 /*
- * workaround on 64-bit macOS which defines size_t as unsigned long and defines
- * uint64_t as unsigned long long, both of which are 8 bytes
+ * workaround on 64-bit macOS and OpenBSD which both define size_t as unsigned
+ * long and define uint64_t as unsigned long long, both of which are 8 bytes
  */
-#if defined(__APPLE__) && defined(__x86_64__)
+#if (defined(__APPLE__) || defined(__OpenBSD__)) && defined(__x86_64__)
 static_assert(sizeof(size_t) == sizeof(uint64_t),
               "Size of size_t and uint64_t are not the same");
 template <>