manpage shouldnt end up in datadir

with e5441c6d

the manpage ends up in $PREFIX/share/man, whilst on openbsd (and maybe other bsds) the manpages are in $PREFIX/man/

apparently mandir is a meson variable that exists, so

Index: doc/meson.build
--- doc/meson.build.orig
+++ doc/meson.build
@@ -4,5 +4,5 @@ custom_target(
   output: 'xfce4-terminal.1',
   command: [xsltproc, '--nonet', '--output', '@OUTPUT@', 'http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl', '@INPUT@'],
   install: true,
-  install_dir: get_option('prefix') / get_option('datadir') / 'man' / 'man1',
+  install_dir: get_option('prefix') / get_option('mandir') / 'man1',
 )

has the expected result here.