disable use of <any> on pre-mojave macos due to missing any_cast ... r…

…esolves #134

CHANGES

@@ -5,6 +5,7 @@ Following is a brief summary of changes made in each release of Libint.
 
 - 2019-xx-yy: 2.6.0-beta.6
   - Ten-no Gm evaluation for m=-1,0 does not use interpolation to guarantee ~1e-14 precision
+  - resolved issue #134: incomplete std::any functionality in libc++ on pre 10.14 macos
 
 - 2019-06-20: 2.6.0-beta.5
   - fixed bundled boost unpacking issues in exported lib CMake build

include/libint2/util/any.h

@@ -27,15 +27,30 @@
 #include <string>
 #include <cassert>
 
-// Include C++17 any header
+// Include C++17 any header, if available AND functional
 #if __cplusplus >= 201703L
-#include <any>
+// macos < 10.14 do not have any_cast in their libc++
+# include <ciso646>  // see https://stackoverflow.com/a/31658120
+# if defined(_LIBCPP_VERSION) && defined(__APPLE__)
+#  include <Availability.h>
+#  ifdef __MAC_OS_X_VERSION_MIN_ALLOWED
+#   if __MAC_OS_X_VERSION_MIN_ALLOWED >= 10140
+#    define LIBINT_HAS_CXX17_ANY
+#   endif  //  10.14 or later
+#  endif  // have macos version
+# else // libc++ on macos
+#  define LIBINT_HAS_CXX17_ANY
+# endif  // libc++ on macos
+#endif  // c++17
+
+#ifdef LIBINT_HAS_CXX17_ANY
+# include <any>
 #endif
 
 namespace libint2 {
 
-// use C++17 std::any, if available
-#if __cplusplus >= 201703L
+// prefer std::any, if available
+#ifdef LIBINT_HAS_CXX17_ANY
 using std::any;
 using std::any_cast;
 using std::bad_any_cast;