autoconf: check all doc deps, fail if unavailable

Check for the breathe extension in the configure script.

Fail if the dependencies are not met, instead of silently disabling
the docs build (build is off by default, and currently not so useful).

Author: jplloyd

configure.ac

@@ -185,10 +185,19 @@ AC_ARG_ENABLE(docs,
 )
 
 if test "x$enable_docs" = xyes; then
+  AC_MSG_NOTICE([Checking requirements for building docs: doxygen, sphinx + breathe extension])
   AC_CHECK_PROGS(DOXYGEN, doxygen)
+  AS_IF(test "x$DOXYGEN" != "x", [], [AC_MSG_ERROR([doxygen is required to build docs])])
   AC_CHECK_PROGS(SPHINX_BUILD, sphinx-build2 sphinx-build-2 sphinx-build3 sphinx-build-3 sphinx-build)
-  # TODO: the python "breathe" extension is also a dependency to doc building.
-  # The configure script should check for its existence.
+  AS_IF(test "x$SPHINX_BUILD" != "x", [], [AC_MSG_ERROR([sphinx is required to build docs])])
+  AC_MSG_CHECKING(for sphinx extension "breathe")
+  AS_IF([dnl
+  python -c "
+import breathe
+import breathe.exception
+breathe.exception.BreatheError" 2> /dev/null],
+  [AC_MSG_RESULT([yes])],
+  [AC_MSG_RESULT([no]); AC_MSG_ERROR(the sphinx breathe extension is required to build docs)])
 fi
 AM_CONDITIONAL(ENABLE_DOCS,
                test "x$DOXYGEN" != "x" && test "x$SPHINX_BUILD" != "x")