Don't install documentation if it isn't built

The HAVE_DOC_GENERATOR variable was always true.

(cherry picked from commit 3d46c1f9)

Conflicts:
	doc/CMakeLists.txt

doc/CMakeLists.txt

@@ -2,7 +2,11 @@
 #  CMake file for OpenCV docs
 #-----------------------
 
-set(HAVE_DOC_GENERATOR BUILD_DOCS AND DOXYGEN_FOUND)
+if(BUILD_DOCS AND DOXYGEN_FOUND)
+  set(HAVE_DOC_GENERATOR TRUE)
+else()
+  set(HAVE_DOC_GENERATOR FALSE)
+endif()
 
 if(HAVE_DOC_GENERATOR)
   project(opencv_docs)