[coreboot] [help]build cbfstool fail with cygwin64

[Julius Werner]

> is there anyway to dynamic define std to gnu99 when detect build with cygwin?

cbfstool is already defining -std=c99. I don't see a strong reason why
we shouldn't just change that to gnu99 globally.

>>> 2. default build will error as below,
>>> HOSTCC     cbfstool/cbfstool.o
>>> /cygdrive/d/FW/coreboot/util/cbfstool/cbfstool.c: In function 'main':
>>> /cygdrive/d/FW/coreboot/util/cbfstool/cbfstool.c:1075:5: error: array
>>> subscript has type 'char' [-Werror=char-subscripts]
>>>      if (tolower(suffix[0])=='k') {
>>>      ^

This sounds like an bug in cygwin and you should file it with them.
They're probably implementing tolower() in their standard header as a
macro which directly indexes an array with an argument. They should at
least be casting the argument inside that macro or they are not
POSIX-compliant.