BLD: require libobjcryst 2017.2.1 or later

When building with libobjcryst require this minimum version.

Author: pavoljuhas

src/diffpy/srreal/ObjCrystStructureAdapter.cpp

@@ -23,8 +23,13 @@
 #include <cmath>
 #include <set>
 
+#include <ObjCryst/version.h>
 #include <diffpy/srreal/ObjCrystStructureAdapter.hpp>
 
+#if LIBOBJCRYST_VERSION < 2017002001000LL
+#error libdiffpy requires libobjcryst 2017.2.1 or later.
+#endif
+
 using namespace std;
 
 namespace diffpy {