Added rudimentary package configuration files.

These files will allow cmake based projects to find the
library using find_package(cJSON). If successful, they can
use CJSON_LIBRARIES and CJSON_INCLUDE_DIRS.

Other build systems can use a similar approach to pkg-config:
cmake --find-package -DNAME=cJSON -DCOMPILER_ID=GNU -DLANGUAGE=C  -DCMAKE_PREFIX_PATH=<path-to-non-sytem-installation-path> -DMODE=LINK

or with MODE COMPILE or EXISTS.

cJSONConfigVersion.cmake.in

@@ -0,0 +1,11 @@
+set(PACKAGE_VERSION "@PROJECT_VERSION@")
+
+# Check whether the requested PACKAGE_FIND_VERSION is compatible
+if("${PACKAGE_VERSION}" VERSION_LESS "${PACKAGE_FIND_VERSION}")
+  set(PACKAGE_VERSION_COMPATIBLE FALSE)
+else()
+  set(PACKAGE_VERSION_COMPATIBLE TRUE)
+  if ("${PACKAGE_VERSION}" VERSION_EQUAL "${PACKAGE_FIND_VERSION}")
+    set(PACKAGE_VERSION_EXACT TRUE)
+  endif()
+endif()