Fix the check for empty string in CMakeLists.txt.

Author: var-const

libcxx/CMakeLists.txt

@@ -74,7 +74,8 @@ set(LIBCXX_ASSERTION_SEMANTIC "" CACHE STRING
   mode in effect. Note that users can override this setting in their own code.
   This does not affect the ABI. Supported values are
   ${LIBCXX_SUPPORTED_ASSERTION_SEMANTICS}.")
-if ("${LIBCXX_ASSERTION_SEMANTIC}" AND NOT "${LIBCXX_ASSERTION_SEMANTIC}" IN_LIST LIBCXX_SUPPORTED_ASSERTION_SEMANTICS)
+
+if (NOT "${LIBCXX_ASSERTION_SEMANTIC}" STREQUAL "" AND NOT "${LIBCXX_ASSERTION_SEMANTIC}" IN_LIST LIBCXX_SUPPORTED_ASSERTION_SEMANTICS)
   message(FATAL_ERROR
     "Unsupported assertion semantic: '${LIBCXX_ASSERTION_SEMANTIC}'. Supported values are ${LIBCXX_SUPPORTED_ASSERTION_SEMANTICS}.")
 endif()