Install the import library via the EXTRA_LIB mechanism on mingw as well

Prior to 3f69873c99, there already was a mechanism for installing
the import library, used for the MSVC builds. Use that for mingw
as well, instead of adding more hardcoded rules.
Set the right SHAREDLIBSUFFIXVER for msvc
Honor the silent build flag when adding the shared library symlink
Fix the dependencies of the install-shared target

Previously calling "make install" failed unless the target
actually already was built, for platforms where the shared
libraries are versioned.
Simplify setting the shlib flags for msvc
Actually install the versioned shared library file as well
Check the suffix instead of checking whether a file is a link

This feels more straightforward - some file systems/environments
(such as MSys) implement symlinks as plain file copies, where
distinguishing whether a file is a link might be hard.
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.
Merge pull request #1582 from mstorsjo/lib-install-fixes

Fix/clean up building and installing shared libraries