[OUTPUT] Make outc an inline function

As "gcc -pedantic" warns, ISO C forbids conditional expressions with
only one void side.  So the (needslow ?  slowpath() : fastpath) code
for outc in the !USE_GLIBC_STDIO case might not be portable.

More importantly, it's hard to read.  Rip it out and replace it
with an inline function which should generate the same code.

Reported-by: Szabolcs Nagy <nsz@port70.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>