Make: add CppDef extra for extracting FOO in -DFOO as a macro of CPreProcessor

[masatake]

See bootlin/elixir#221 .

input.mak:

CPPFLAGS = -DOUTPUT=stdout
foo.o: foo.c
	$(CC) $(CPPFLAGS) $<

Output:

$ ./ctags --extras-Make='+{CppDef}' --fields=+lE -o - input.mak 
CPPFLAGS	input.mak	/^CPPFLAGS = -DOUTPUT=stdout$/;"	m	language:Make
OUTPUT	input.mak	/^CPPFLAGS = -DOUTPUT=stdout$/;"	d	language:CPreProcessor	extras:CppDef
foo.o	input.mak	/^foo.o: foo.c$/;"	t	language:Make

When --extras-Make='+{CppDef}' is given, the Make parser extracts OUTPUT.

Violent heuristics, but I know this is useful when reading a source tree like Linux kernel.
I have tried hard to make the impact on the original code small.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.45%. Comparing base (ac6c14c) to head (c16adc6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4024      +/-   ##
==========================================
+ Coverage   85.42%   85.45%   +0.03%     
==========================================
  Files         235      235              
  Lines       56729    56848     +119     
==========================================
+ Hits        48462    48582     +120     
+ Misses       8267     8266       -1     

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[masatake]

We must merge #4025 first.
The coverage must be 100%.

[masatake]

The last one line the test cases don't cover looks a bit strange for me. Though I have modified the line, I cannot convince we need the line.
I took more time for inspecting the line before merging.