Check that aligned_alloc is available with cmake

Some compilers support C++17 even on targets that lack required functions.
Projects that want to force C++17 will then run into a problem with
std::aligned_alloc not existing on those targets, so it needs to be explicitly
checked for. The alternative is to simply never use it even when it would be
available.

common/almalloc.cpp

@@ -17,7 +17,7 @@ void *al_malloc(size_t alignment, size_t size)
     assert((alignment & (alignment-1)) == 0);
     alignment = std::max(alignment, alignof(std::max_align_t));
 
-#if __cplusplus >= 201703L
+#if defined(HAVE_STD_ALIGNED_ALLOC)
     size = (size+(alignment-1))&~(alignment-1);
     return std::aligned_alloc(alignment, size);
 #elif defined(HAVE_POSIX_MEMALIGN)
@@ -48,7 +48,7 @@ void *al_calloc(size_t alignment, size_t size)
 
 void al_free(void *ptr) noexcept
 {
-#if (__cplusplus >= 201703L) || defined(HAVE_POSIX_MEMALIGN)
+#if defined(HAVE_STD_ALIGNED_ALLOC) || defined(HAVE_POSIX_MEMALIGN)
     std::free(ptr);
 #elif defined(HAVE__ALIGNED_MALLOC)
     _aligned_free(ptr);