MXS-1220: Always build libmicrohttpd

This allows MaxScale to use the PATCH functionality that's only in the
newer versions of the library. It also removes some of the problems that
exist with older systems that don't support all of the options.

CMakeLists.txt

@@ -71,6 +71,8 @@ if(NOT JANSSON_FOUND)
   include(cmake/BuildJansson.cmake)
 endif()
 
+include(cmake/BuildMicroHttpd.cmake)
+
 include_directories(${JANSSON_INCLUDE_DIR})
 
 # You can find the variables set by this in the FindCURL.cmake file