Respect CMake install paths

[aperechnev]

There is CMakeLists.txt file with an absolute installation path:

install(TARGETS monero-wallet-gui
  DESTINATION bin
)

where bin usually points to /usr/local/bin.

It works well, but if some build system attempts to produce a package, it will fail since a user might not have root privileges. Such build systems usually place artifacts in a dedicated directory, like ~/my-package/usr/local/bin/. As an example, FreeBSD's Makefile and Poudriere always fail during the insufficient access rights. I believe this is not the only case.

So I just changed the installation path to respect CMake prefixes according to CMake documentation:

install(TARGETS monero-wallet-gui
  DESTINATION ${CMAKE_INSTALL_BINDIR}
)

And now the produced executable can be placed to the right directory.