enable c++20 (#173)

Adds checks for whether the toolchain in use is using libc++ and if
it only has an experimental version of memory_resource

Essentially this is necessary to support building c++17/20 with older clangs using libc++.

CheckExperimentalMemoryResource.cmake

@@ -0,0 +1,16 @@
+include(CheckCXXSourceCompiles)
+
+# The libc++ implementation that ships with clang < v16 still
+# has the memory_resource header under experimental.
+#
+# This functions checks if we are using such a standard library
+# implementation.
+#
+function(check_experimental_memory_resource success)
+  check_cxx_source_compiles("
+#include <experimental/memory_resource>
+  int main() {
+    return 0;
+  }
+  " ${success})
+endfunction()

CheckLibcpp.cmake

@@ -0,0 +1,18 @@
+include(CheckCXXSourceCompiles)
+
+# Checks if libc++ is being used
+#
+function(check_libcpp success)
+  check_cxx_source_compiles("
+  #include <iostream>
+  int a =
+  #ifdef _LIBCPP_VERSION
+    1;
+  #else
+    kdfasfdl
+  #endif
+  int main() {
+    return 0;
+  }
+  " ${success})
+endfunction()