migrate from directory level cmake commands to target commands. (#802)

migrate from directory level cmake commands to target commands. (#802)

* migrate from directory level cmake commands to target commands.

fix a few bugs with our CMakeLists:

set WIN32 for gui target.  remove _CONSOLE, it's obsolete.

when undefining UNICODE, also undefine _UNICODE.  it's not clear to
me we have a sensitivity to UNICODE/_UNICODE.

take optimization flags out of CMakeLists, rely on CMAKE_BUILD_TYPE
to select appropriate flags.

don't force ZLIB_INHIBITED to be undefined on windows.

add cmake support for some of our configuration options:
disable-mappreview, embed_map, embed_translations.

* fix duplicated code in gui/CMakeLists.txt

* spelling fix for INSTALL.