Fix Errno.pm generation for gcc-5.0

Fix Errno.pm generation for gcc-5.0

gcc-5.0 -E interleaves now line numbers with expended macros, so that
the generated errno.c will be preprocessed to

EBFONT => [[
           59
                ]]

which is hard to parse in in line-based reader.

So use -P option with gcc >= 5.0. Global -P usage would break makedepend,
global -ftrack-macro-expansion=0 would break lib/h2ph.t.

RT#123784