Add missing file config.h.cmake

The dune-elasticity module dates from an era where that file
was not needed.

config.h.cmake

+/* begin dune-elasticity
+   put the definitions for config.h specific to
+   your project here. Everything above will be
+   overwritten
+*/
+
+/* begin private */
+/* Name of package */
+#define PACKAGE "@DUNE_MOD_NAME@"
+
+/* Define to the address where bug reports for this package should be sent. */
+#define PACKAGE_BUGREPORT "@DUNE_MAINTAINER@"
+
+/* Define to the full name of this package. */
+#define PACKAGE_NAME "@DUNE_MOD_NAME@"
+
+/* Define to the full name and version of this package. */
+#define PACKAGE_STRING "@DUNE_MOD_NAME@ @DUNE_MOD_VERSION@"
+
+/* Define to the one symbol short name of this package. */
+#define PACKAGE_TARNAME "@DUNE_MOD_NAME@"
+
+/* Define to the home page for this package. */
+#define PACKAGE_URL "@DUNE_MOD_URL@"
+
+/* Define to the version of this package. */
+#define PACKAGE_VERSION "@DUNE_MOD_VERSION@"
+
+/* end private */
+
+/* Define to the version of dune-elasticity */
+#define DUNE_ELASTICITY_VERSION "@DUNE_ELASTICITY_VERSION@"
+
+/* Define to the major version of dune-elasticity */
+#define DUNE_ELASTICITY_VERSION_MAJOR @DUNE_ELASTICITY_VERSION_MAJOR@
+
+/* Define to the minor version of dune-elasticity */
+#define DUNE_ELASTICITY_VERSION_MINOR @DUNE_ELASTICITY_VERSION_MINOR@
+
+/* Define to the revision of dune-elasticity */
+#define DUNE_ELASTICITY_VERSION_REVISION @DUNE_ELASTICITY_VERSION_REVISION@
+
+/* end dune-elasticity
+   Everything below here will be overwritten
+*/