talkatu/talkatu

Added meson option to select if the demo application will be installed or not

Testing Done:
1. Compiled, installed with the default options and verified that the demo application and its manpage weren't installed.

2. Deleted `build` directory, reconfigured the install option added in this MR with `meson configure -Dinstall-demo=true` and then compiled and installed. This time the demo application and its manpage were installed in my system.

Bugs closed: TALKATU-84

Reviewed at https://reviews.imfreedom.org/r/156/
/*
* talkatu
* Copyright (C) 2017-2019 Gary Kramlich <grim@reaperworld.com>
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, see <https://www.gnu.org/licenses/>.
*/
#if !defined(TALKATU_GLOBAL_HEADER_INSIDE) && !defined(TALKATU_COMPILATION)
#error "only <talkatu.h> may be included directly"
#endif
#ifndef TALKATU_CORE_H
#define TALKATU_CORE_H
G_BEGIN_DECLS
void talkatu_init(void);
void talkatu_uninit(void);
G_END_DECLS
#endif /* TALKATU_CORE_H */