Fix EXTERNAL_LIBC_INTERFACE check with USE_COMPILER_DEFAULT_LIBC

Related with #331

JerryScript-DCO-1.0-Signed-off-by: SaeHie Park [email protected]

Author: seanshpark

CMakeLists.txt

@@ -93,7 +93,7 @@ project (Jerry CXX C ASM)
 
 # Should we use external libc?
  if(DEFINED USE_COMPILER_DEFAULT_LIBC AND USE_COMPILER_DEFAULT_LIBC STREQUAL "YES")
-  if(NOT EXTERNAL_LIBC_INTERFACE STREQUAL "" AND NOT EXTERNAL_LIBC_INTERFACE STREQUAL "UNDEFINED")
+  if(DEFINED EXTERNAL_LIBC_INTERFACE AND NOT EXTERNAL_LIBC_INTERFACE STREQUAL "UNDEFINED")
     message(FATAL_ERROR "EXTERNAL_LIBC_INTERFACE='${EXTERNAL_LIBC_INTERFACE}' should not be set in case compiler's default libc is used (USE_COMPILER_DEFAULT_LIBC=YES)")
   endif()