cmake: significant changes to accomodate a new "bundled" mode.

This makes it much easier to include LZ4 in a CMake-based project;
you only need to add_subdirectory(lz4/contrib/cmake_unofficial),
then you can use the lz4 target (which is the library).  In bundled
mode a static library will be built and nothing will be installed,
although the lz4 and lz4c command line tools will still be built.

This also makes some other much less intrusive changes, including:

 * Version information is now parsed from lib/lz4.h so the
   CMakeLists.txt file no longer needs to manually be kept up to date
   manually.
 * pkg-config file uses ${prefix} where appropriate instead of writing
   out full paths, which is important for people on Windows.
 * Warning flags are now checked instead of blindly added.
 * The GNUInstallDirs module is used to determine installation
   locations.
 * The static library generates position independent code, which means
   it's usable when building a shared library.
 * The CLI tools link to liblz4 instead of duplicating the object code.

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