Added option to use a system inchi library.

This is my first attempt at adding an option to build against a system inchi
library. It looks for an inchi library in the standard locations, and adds in
some logic to use that library if selected.

cmake/modules/FindInchi.cmake

@@ -0,0 +1,23 @@
+# - Try to find Inchi lib
+# Once done this will define
+#
+#  INCHI_FOUND - system has eigen lib
+#  INCHI_INCLUDE_DIR - the eigen include directory
+#  INCHI_LIBRARIES - the inchi library
+
+# Copyright (c) 2010 Marcus D. Hanwell, <marcus@cryos.org>
+# Redistribution and use is allowed according to the terms of the BSD license.
+# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
+
+if(INCHI_INCLUDE_DIR AND INCHI_LIBRARY)
+  # in cache already
+  set(INCHI_FOUND TRUE)
+else()
+  find_path(INCHI_INCLUDE_DIR NAMES inchi_api.h)
+  find_library(INCHI_LIBRARY NAMES inchi Inchi)
+  include(FindPackageHandleStandardArgs)
+  find_package_handle_standard_args(INCHI DEFAULT_MSG INCHI_LIBRARY
+    INCHI_INCLUDE_DIR)
+  mark_as_advanced(INCHI_INCLUDE_DIR INCHI_LIBRARIES)
+endif()
+