[mlpl][test][Makefile] Integrate compiler flags defined in 'INCLUDE' into AM_CXXFLAGS to suppress a warning.

A newer automaker (e.g. 1.13.3) shows the following the warning.

server/mlpl/test/Makefile.am:18: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')