doc: Re-introduced VERSION parsing into generated docs

Will now grab version from /VERSION and put it into Doxyfile before
making the documentation.
diff --git a/doc/Doxyfile b/doc/Doxyfile.in
similarity index 99%
rename from doc/Doxyfile
rename to doc/Doxyfile.in
index 4f56753..0714b6d 100644
--- a/doc/Doxyfile
+++ b/doc/Doxyfile.in
@@ -38,7 +38,7 @@
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 2.1.0-pre
+PROJECT_NUMBER         = LIBSRTPVERSIONNUMBER
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a
diff --git a/doc/Makefile.in b/doc/Makefile.in
index 7aedffd..5b22fb3 100644
--- a/doc/Makefile.in
+++ b/doc/Makefile.in
@@ -18,14 +18,15 @@
 
 .PHONY: libsrtpdoc clean
 libsrtpdoc:
-	@if test ! -e Doxyfile; then \
+	@if test ! -e Doxyfile.in; then \
 		echo "*** Sorry, can't build doc outside source dir"; exit 1; \
 	fi
+	sed 's/LIBSRTPVERSIONNUMBER/$(version)/' Doxyfile.in > Doxyfile
 	doxygen
 
 clean:
 
-	rm -rf html/ xml/
+	rm -rf html/ xml/ Doxyfile
 	for a in * ; do							\
 			  if [ -f "$$a~" ] ; then rm -f $$a~; fi;	\
 		done;