ENH: Added CHECK_VARIABLE_EXISTS macro.

Modules/CheckVariableExists.c

@@ -0,0 +1,16 @@
+#ifdef CHECK_VARIABLE_EXISTS
+
+extern int CHECK_VARIABLE_EXISTS;
+
+int main()
+{
+  int* p;
+  p = &CHECK_VARIABLE_EXISTS;
+  return 0;
+}
+
+#else  /* CHECK_VARIABLE_EXISTS */
+
+#  error "CHECK_VARIABLE_EXISTS has to specify the variable"
+
+#endif /* CHECK_VARIABLE_EXISTS */

Modules/CheckVariableExists.cmake

@@ -0,0 +1,23 @@
+#
+# Check if the variable exists.
+# # CHECK_VARIABLE_EXISTS - macro which checks if the variable exists
+# VAR - the name of the variable
+# VARIABLE - variable to store the result
+#
+
+MACRO(CHECK_VARIABLE_EXISTS VAR VARIABLE)
+  SET(MACRO_CHECK_VARIABLE_DEFINITIONS -DCHECK_VARIABLE_EXISTS=${VAR})
+  TRY_COMPILE(${VARIABLE}
+             ${PROJECT_BINARY_DIR}
+             ${CMAKE_ROOT}/Modules/CheckVariableExists.c
+             CMAKE_FLAGS -DCOMPILE_DEFINITIONS:STRING=${MACRO_CHECK_VARIABLE_DEFINITIONS}
+             OUTPUT_VARIABLE OUTPUT)
+  IF(${VARIABLE})
+    SET(${VARIABLE} 1 CACHE INTERNAL "Have variable ${VAR}")
+  ELSE(${VARIABLE})
+    SET(${VARIABLE} "" CACHE INTERNAL "Have variable ${VAR}")
+    WRITE_FILE(${PROJECT_BINARY_DIR}/CMakeError.log 
+      "Determining if the variable ${VAR} exists failed with the following output:\n"
+      "${OUTPUT}\n" APPEND)
+  ENDIF(${VARIABLE})
+ENDMACRO(CHECK_VARIABLE_EXISTS)