allow static and shared builds from top level

Makefile

 default: linux
 
-linux:
-	$(MAKE) -C src
-	$(MAKE) -C example
+linux: static shared example
+
+macos: linux
+
+static:
+	$(MAKE) -C src static
+
+shared:
+	$(MAKE) -C src shared
 
-macos:
-	$(MAKE) -C src
+example:
 	$(MAKE) -C example
 
 win32: