Add FindJansson.cmake

Added find_package module for CMake which locates the Jansson libraries
and headers. This will make dependency checking easier and prevents build
failures due to missing dependencies.

cmake/FindJansson.cmake

@@ -0,0 +1,16 @@
+# This CMake file locates the Jansson libraries and headers
+#
+# The following variables are set:
+# JANSSON_FOUND - If the Avro C library was found
+# JANSSON_LIBRARIES - Path to the static library
+# JANSSON_INCLUDE_DIR - Path to Avro headers
+
+find_path(JANSSON_INCLUDE_DIR jansson.h)
+find_library(JANSSON_LIBRARIES NAMES libjansson.a libjansson.so)
+
+if (JANSSON_INCLUDE_DIR AND JANSSON_LIBRARIES)
+  message(STATUS "Found Jansson: ${JANSSON_LIBRARIES}")
+  set(JANSSON_FOUND TRUE)
+else()
+  message(STATUS "Could not find Jansson")
+endif()