Build shared libraries by default

Now internal libraries are build as shared ones to reduce linking
time.

One can build static libraries using '-DBUILD_SHARED_LIBS=OFF'

CMakeLists.txt

@@ -79,6 +79,12 @@ if(NOT CMAKE_BUILD_TYPE)
       FORCE)
 endif()
 
+#------------------------------------------------------
+# default build shared libraries
+if (NOT BUILD_SHARED_LIBS)
+  set(BUILD_SHARED_LIBS ON CACHE STRING "" FORCE)
+endif()
+
 #------------------------------------------------------
 
 # Checks if compiler version is compatible with Pugs sources