If enable_build_docs is "no", we don't even try to look for doxygen.

Also set BUILD_DOCS in that branch. Now the disable-build-docs works
as expected.

acinclude.m4

@@ -50,6 +50,10 @@ AC_DEFUN([AM_CHECK_DOXYGEN],
            enable_build_docs="yes"
         fi],
        [enable_build_docs="yes"])
+
+    if test "$enable_build_docs" = "no" ; then
+        BUILD_DOCS=no
+    else
     dnl
     dnl Get the prefix where doxygen is installed.
     dnl
@@ -93,6 +97,7 @@ AC_DEFUN([AM_CHECK_DOXYGEN],
            AC_MSG_WARN(
               [Warning: no doxygen detected. Documentation will not be built])
         fi])
+    fi
     AC_MSG_CHECKING([whether documentation is built])
     AC_MSG_RESULT([${BUILD_DOCS}])
     dnl