Add SLEEF_BUILD_SHARED_LIBS to override BUILD_SHARED_LIBS from including projects (#531)

* Set the CMake minimum version and project as early as possible

Also uses the standard version variables. Replaced PATCHLEVEL by
PATCH except in #define'd macros (to preserve API).

Replaced languages list with enable_language.

Removed policy options that do not affect runtime (because they
are enabled by the minimum version, anyway)

* Add SLEEF_BUILD_SHARED_LIBS to override BUILD_SHARED_LIBS from including projects.

* Remove "dummy" install targets

* Use ON/OFF for the normalized values of BUILD_SHARED_LIBS