The new build flag STP_USE_LIB can be set to choose the STP library to be used.
Options are '', 'included', 'system', 'disable', and an absolute path starting with '/'.

The empty string defaults to the same behavior as 'included' which builds STP from the provided source.
'disable' has the same effect as STP_STUB=1, i.e. disabling STP at runtime.
'system' tries to find the system library (currently only in /usr/lib) and then copies it for building.
An absolute path starting with '/' can be provided and must point to the directory in which the desired STP library is located. Otherwise the
same as 'system'.

The options for precompiled STP libraries essentially do the same things as STP_STUB, except they copies the library
instead of compiling the stub. For compatibility reasons within the Makefiles, the library is linked to under
the soname libstp.so.1 eventhough technically it should be libstp.so.2.3 (for the newest versions), but STP is backwards-compatible
enough that this does not cause issues.

Enabling the flag reduced the build time by up to 25% (the time to compile STP from source), while `make TEST_BSC_OPTIONS="-sat-stp" smoke`
saw no reductions, both compared to the provided source and Yices.

Defining STP_STUB now creates an error with a hint to use STP_USE_LIB=disable. The new way is
more readable and clearer in intent.
Since STP_STUB had a bug that made it effectively unusable for 3 years, replacing the option
likely affects few users while those that worked around the bug have to change things anyway.

This change also enabled some minor refactoring work for better readability.

On MacOS, libraries have a different suffix and the filesystem is structured differently.
This commit adds the relevant bits and pieces for the option STP_USE_LIB to work on MacOS
in principle.

Also fixes the issue with STP_STUB (which is used with STP_USE_LIB=disable), where the wrong
library file is copied, resulting in a linking error down the line.

On Linux, use find in /usr/lib to find the correct library. Only consider the first result;
all results should point to the same source anyway.
The results are also cleaned to ensure reliable behavior (the library name must only contain
alphanumerics + colon)

Improve cleaning of user input when a path is passed in

The logic for STP_USE_LIB has been indented to improve the readability and
make the different cases more obvious.

Also, minor fixes. `find` ends with NUL byte, so replace that with newline.
The BSC linker needs at least a symlink named libstp.so
The SNAME is completely independent from the library name, so just choose a fitting one

The SNAME is exported in the general stp Makefile. The symbolic link is created there too.
Remove both actions from the special case as they are handled in the general case.

Using STP_USE_LIB=placeholder means that the stub is compiled with a generic name and
copied to the installation, but is supposed to be replaced at a later date.

To make the intentions clear, the library is renamed to *_placeholder with a symlink
created with the original name pointing to the library.