Remove dependencies on Boost.StaticAssert. (#229)

Boost.StaticAssert has been merged into Boost.Config, so remove
the dependency.

Author: Lastique

test/cmake_subdir_test/CMakeLists.txt

@@ -14,7 +14,6 @@ add_subdirectory(../../../detail boostorg/detail)
 add_subdirectory(../../../describe boostorg/describe)
 add_subdirectory(../../../integer boostorg/integer)
 add_subdirectory(../../../mp11 boostorg/mp11)
-add_subdirectory(../../../static_assert boostorg/static_assert)
 add_subdirectory(../../../throw_exception boostorg/throw_exception)
 add_subdirectory(../../../type_traits boostorg/type_traits)