Make Arduino.mk compatible with Flymake

If Flymake is configured to parse .ino files the same way as for c/c++
files, it creates a temporary file (_flymake.ino) in the same directory
as the original file. It fails with the current Arduino.mk because of
the check for multiple .ino files. This fix removes the check
only when flymake is calling the Makefile (Flymake will call make with
the variable CHK_SOURCES set to the temporary file name)

To make Flymake working with .ino file :

Add :
check-syntax:
	$(CXX_NAME) -c -include Arduino.h   -x c++ $(CXXFLAGS)   $(CPPFLAGS)  -fsyntax-only $(CHK_SOURCES)

in the project Makefile after the inclusion of the Arduino.mk file

Edit the flymake configuration :
M-x customize-option RET
flymake-allowed-file-name-masks RET (using auto completion !)

Add the line :

("\\.ino\\'" flymake-simple-make-init)

Then click on "Apply and Save" button