Handle generating the versionless library via make rules

This makes sure both "make libopenh264.so" and "make libopenh264.so.0"
work as intended.

Use the versionless name as dependency for the install rule,
to make sure both (if possible) are created.
Set SHAREDLIB_DIR for msvc as well
Actually install the shared library into $(SHAREDLIB_DIR) as intended
Simplify creating output directories for the install-shared rule
Set a default SHAREDLIB_DIR in the main Makefile

Only override it in the ones where the default isn't desired.