contrib/zlib: disable dynamic library building

Fixes compile failure for static only toolchains (and assimp
links against the static one).

contrib/zlib/CMakeLists.txt

@@ -196,10 +196,7 @@ if(MINGW)
     set(ZLIB_DLL_SRCS ${CMAKE_CURRENT_BINARY_DIR}/zlib1rc.obj)
 endif(MINGW)
 
-add_library(zlib SHARED ${ZLIB_SRCS} ${ZLIB_ASMS} ${ZLIB_DLL_SRCS} ${ZLIB_PUBLIC_HDRS} ${ZLIB_PRIVATE_HDRS})
 add_library(zlibstatic STATIC ${ZLIB_SRCS} ${ZLIB_ASMS} ${ZLIB_PUBLIC_HDRS} ${ZLIB_PRIVATE_HDRS})
-set_target_properties(zlib PROPERTIES DEFINE_SYMBOL ZLIB_DLL)
-set_target_properties(zlib PROPERTIES SOVERSION 1)
 
 INSTALL( TARGETS zlibstatic
          LIBRARY DESTINATION ${ASSIMP_LIB_INSTALL_DIR}