cmake: Sync txmg behavior with configure.

Discovered due to -DENABLE_ADOPTED_EXPERIMENTS configure/cmake
desync check. Desynced since g783d7e11.

BUG=aomedia:856

Change-Id: If5c61126d89c517a511c51a8a1ee2f0862ddb96f

build/cmake/aom_configure.cmake

@@ -56,8 +56,9 @@ if (NOT ENABLE_ADOPTED_EXPERIMENTS)
     unset(var_helpstring)
     get_property(var_helpstring CACHE ${var} PROPERTY HELPSTRING)
     if ("${var_helpstring}" STREQUAL "AV1 experiment flag.")
-     if ("${var}" STREQUAL "CONFIG_CB4X4")
+      if ("${var}" MATCHES "CONFIG_CB4X4\|CONFIG_TXMG")
        # CB4X4 is required and can not be disabled.
+       # TXMG is required for configure compatibility.
      else ()
        set(${var} 0)
      endif ()