[warnigectomy] Use -Werror=... rather than --std=c89

On win32 we still need c89/c90 w/o gnu extension due to msvc6 use.
But --std=c89 is very strict, e.g. it prohibits single line
comments.
The following -Werror= reject constructs which break the Windows
build

 -Werror=declaration-after-statement
 -Werror=implicit-function-declaration

There might be more conmstructs to avoid, but these arte the most
pressing. Actually the second is turned into an error explicetly
on Windows, too. But the first one is a compiler limitation.
[warningectomy] avoid fwrite() unused result warning

Noise reduction for
return value of 'fwrite', declared with attribute warn_unused_result
discussion: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509
root cause: http://sourceware.org/bugzilla/show_bug.cgi?id=11959