Fix build with CMake < 3.0

cmake/CXX11Check.cmake

@@ -56,7 +56,7 @@ function(check_cxx11 CHECK RESULTVAR)
 			set(file "${CXX11_CHECK_DIR}/check-cxx11-${check}.cpp")
 			check_compile(result "${file}" "${CHECK}" "C++11 feature")
 		endif()
-		if(result STREQUAL "")
+		if(NOT DEFINED result OR result STREQUAL "")
 			set(${RESULTVAR} OFF PARENT_SCOPE)
 		else()
 			set(${RESULTVAR} ON PARENT_SCOPE)

cmake/CompileCheck.cmake

@@ -17,6 +17,10 @@
 #    misrepresented as being the original software.
 # 3. This notice may not be removed or altered from any source distribution.
 
+# Note: In CMake before 3.0 set(var "" PARENT_SCOPE) *unsets* the variable in the
+# parent scope instead of setting it to the empty string.
+# This means if(var STREQUAL "") will be false since var is not defined and thus not expanded.
+
 function(check_compile RESULT FILE FLAG TYPE)
 	
 	string(REGEX REPLACE "[^a-zA-Z0-9_][^a-zA-Z0-9_]*" "-" cachevar "${TYPE}-${FLAG}")
@@ -142,7 +146,7 @@ function(add_cxxflag FLAG)
 	
 	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${RESULT}" PARENT_SCOPE)
 	
-	if(RESULT STREQUAL "")
+	if(NOT DEFINED RESULT OR RESULT STREQUAL "")
 		set(FLAG_FOUND 0 PARENT_SCOPE)
 	else()
 		set(FLAG_FOUND 1 PARENT_SCOPE)
@@ -158,7 +162,7 @@ function(add_ldflag FLAG)
 	set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${RESULT}" PARENT_SCOPE)
 	set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${RESULT}" PARENT_SCOPE)
 	
-	if(RESULT STREQUAL "")
+	if(NOT DEFINED RESULT OR RESULT STREQUAL "")
 		set(FLAG_FOUND 0 PARENT_SCOPE)
 	else()
 		set(FLAG_FOUND 1 PARENT_SCOPE)