Modernise CMake

* Use `GNUInstallDirs` in order to respect GNU conventions.
  This is especially important for multi-arch/multi-lib setups.
* Specify position independent mode building properly, without
  using the historic hack of adding `-fPIC` as a definition.
  This makes the build system more portable.
* Only detect C++ (and not C) to slightly speed up configuring.
* Specify C++11 mode using modern CMake idioms.
* Fix some whitespace issues.