more defensive check for COMPILER_ID variable

CMake/cotire.cmake

@@ -2643,10 +2643,12 @@ function (cotire_choose_target_languages _target _targetLanguagesVar _wholeTarge
 			set (${_targetLanguagesVar} "" PARENT_SCOPE)
 			return()
 		endif()
-		if (_targetUsePCH AND "${_language}" MATCHES "^C|CXX$" AND NOT "${CMAKE_${_language}_COMPILER_ID}" STREQUAL "")
-			cotire_check_precompiled_header_support("${_language}" "${_target}" _disableMsg)
-			if (_disableMsg)
-				set (_targetUsePCH FALSE)
+		if (_targetUsePCH AND "${_language}" MATCHES "^C|CXX$" AND DEFINED CMAKE_${_language}_COMPILER_ID)
+			if (CMAKE_${_language}_COMPILER_ID)
+				cotire_check_precompiled_header_support("${_language}" "${_target}" _disableMsg)
+				if (_disableMsg)
+					set (_targetUsePCH FALSE)
+				endif()
 			endif()
 		endif()
 		set (_sourceFiles "")