add doxygen error message and documentation

CMakeLists.txt

@@ -204,4 +204,6 @@ find_package(Doxygen)
 if(DOXYGEN_FOUND)
   configure_file(${CMAKE_SOURCE_DIR}/docs/Doxyfile.in ${CMAKE_BINARY_DIR}/Doxyfile @ONLY)
   add_custom_target(docs ${DOXYGEN_EXECUTABLE} ${CMAKE_BINARY_DIR}/Doxyfile WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
+else()
+  add_custom_target(docs COMMAND ${CMAKE_COMMAND} -E echo "Can't create docs, doxygen not installed \(try sudo apt-get install doxygen on Ubuntu and then rerun cmake\)" VERBATIM)
 endif()

docs/INSTALLING.md

@@ -278,6 +278,18 @@ CXX=g++ CC=gcc PATH=~/pyston_deps/gcc-4.8.2-install/bin:$PATH:~/pyston_deps/ninj
 ~/pyston_deps/ninja/ninja # runs in parallel
 ```
 
+### doxygen
+```
+sudo apt-get install doxygen
+
+# then run cmake (see below) and invoke the docs target
+ninja docs
+
+# now within the Pyston build directory open docs/html/index.html with a browser
+```
+
+Generate doxygen documentation for Pyston. Requires using the cmake build system.
+
 
 # (Experimental) CMake build system