Fix silent builds of the shared library
Include the x86 common header for x86 MSVC as well
Install the shared library as well
Use CXX_LINK_O when building the shared library
Make the -shared flag an overrideable variable
Support building DLLs with MSVC

MSVC doesn't export any functions at all by default, contrary to
gcc which exports all global functions by default.

Doing the same for linux/OS X isn't too hard, but they use yet
two other, different syntaxes for the corresponding symbol file.
Install the import library as well, in MSVC builds
Merge pull request #313 from mstorsjo/fix-msvc-build

Fix msvc build
Merge remote-tracking branch 'upstream/master' into travis.osx