Change Makefile to differentiate between static and shared library

Previously, just the static libraries (libwren*.a) were created. In the Makefile documentation these were mentioned in the comments as shared.
Now both variants will be created.

.gitignore

@@ -8,6 +8,8 @@ wrend
 wren-cpp
 libwren.a
 libwrend.a
+libwren.so
+libwrend.so
 libwren-cpp.a
 
 # XCode user-specific stuff.
@@ -23,4 +25,4 @@ benchmark/baseline.txt
 
 # built docs get copied here, which is presumed to be a separate checkout of
 # the repo so they can be pushed to GitHub Pages.
-gh-pages/
+gh-pages/