Fix EMX_MAKEFILE (adjust path to header files, add -I option)

Fix EMX_MAKEFILE (adjust path to header files, add -I option)

* EMX_MAKEFILE: Remove comment about renaming to Makefile.
* EMX_MAKEFILE (CFLAGS, SPECIALCFLAGS): Add "-I include" option.
* EMX_MAKEFILE (CXXFLAGS): New variable (set to CFLAGS value).
* EMX_MAKEFILE (CORD_INCLUDE_FILES, test.o, setjmp_test.exe): Adjust
path to gc.h.
* EMX_MAKEFILE (SHELL): Remove special variable (not needed for EMX).
* EMX_MAKEFILE (test.o): Adjust path to dependencies (gc_priv.h,
gc_hdrs.h, gcconfig.h, gc_typed.h).
* EMX_MAKEFILE (gc_cpp.o, c++): Adjust path to gc_cpp.h.
* EMX_MAKEFILE (gc_cpp.o): Use CXXFLAGS value instead of "-O".
* EMX_MAKEFILE (c++): Do not copy gc_cpp.h to "include" folder.