Allow shared builds of libwebm.

Omit the STATIC in the add_library command, and stop
forcing static MSVC runtimes when BUILD_SHARED_LIBS
is enabled.

$ cmake path/to/libwebm -DBUILD_SHARED_LIBS=1 && cmake --build .

Change-Id: I4935c774d98f70a39363f37be4f9e8989a0749c0
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ba42a7d..9cc4fe4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,7 +9,10 @@
 project(LIBWEBM CXX)
 
 include("${CMAKE_CURRENT_SOURCE_DIR}/build/cxx_flags.cmake")
-include("${CMAKE_CURRENT_SOURCE_DIR}/build/msvc_runtime.cmake")
+
+if (NOT BUILD_SHARED_LIBS)
+  include("${CMAKE_CURRENT_SOURCE_DIR}/build/msvc_runtime.cmake")
+endif ()
 
 set(LIBWEBM_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
 
@@ -314,7 +317,7 @@
 add_library(mkvparser OBJECT ${mkvparser_sources})
 add_library(webvtt_common OBJECT ${webvtt_common_sources})
 
-add_library(webm STATIC ${libwebm_common_sources}
+add_library(webm ${libwebm_common_sources}
             $<TARGET_OBJECTS:mkvmuxer>
             $<TARGET_OBJECTS:mkvparser>)