ENH: added new test for out of dir source trees

Tests/OutOfSource/CMakeLists.txt

@@ -0,0 +1,3 @@
+# a simple test case
+PROJECT (OutOfSource)
+SUBDIRS(SubDir)

Tests/OutOfSource/OutOfSourceSubdir/CMakeLists.txt

@@ -0,0 +1 @@
+ADD_EXECUTABLE (simple simple.cxx)

Tests/OutOfSource/OutOfSourceSubdir/simple.cxx

@@ -0,0 +1,4 @@
+int main ()
+{
+  return 0;
+}

Tests/OutOfSource/SubDir/CMakeLists.txt

@@ -0,0 +1,2 @@
+# subdir to a sibling dir
+SUBDIRS(${PROJECT_SOURCE_DIR}/OutOfSourceSubdir)