doc/Makefile: ensure C locale
The man pages can actually be translated by building them in a
different locale. However, the man pages contain internal links to
certain sections, and when the section titles are translated, the
links change too. So it is currently not recommended to build the man
pages in anything by the "C" locale.
adding he
llo
abort: '\n' and '\r' disallowed in filenames: 'he\rllo'
adding he
llo
abort: '\n' and '\r' disallowed in filenames: 'he\rllo'
adding hell
o
abort: '\n' and '\r' disallowed in filenames: 'hell\no'
adding hell
o
abort: '\n' and '\r' disallowed in filenames: 'hell\no'
f he
llo he
llo
f hell
o hell
o