autotools: Fix the manpages flag in configure.ac
autotools: Fix the manpages flag in configure.ac

Running ./configure --disable-man-pages is invalid, despite being
reported as valid by the configure script. The actual flag should
be --disable-manpages. This commit corrects this behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=707452

stefwalter/libsecret,GNOME/libsecret,GNOME/libsecret,stefwalter/libsecret,stefwalter/libsecret,stefwalter/libsecret,GNOME/libsecret,stefwalter/libsecret,GNOME/libsecret