autotools: disable doxygen by default

configure.ac

@@ -14,11 +14,11 @@ AC_PROG_CXX
 AM_PROG_AR
 
 dnl Check for Doxygen
-AC_ARG_ENABLE([doc], [AS_HELP_STRING([--disable-doc], [Disable documentation generation (doxygen)])])
-AS_IF([test "x$enable_doc" != "xno"], [
+AC_ARG_ENABLE([doc], AS_HELP_STRING([--enable-doc], [Enable documentation generation (doxygen)]))
+AS_IF([test "x$enable_doc" = "xyes"], [
        AC_CHECK_PROGS([DOXYGEN], [doxygen])
-       AS_IF([test -z "$DOXYGEN"],[AC_MSG_WARN([Doxygen not found - continuing without Doxygen support])])
-       ])
+       AS_IF([test -z "$DOXYGEN"], [AC_MSG_WARN([Doxygen not found - continuing without Doxygen support])])       
+])
 AM_CONDITIONAL([HAVE_DOXYGEN], [test -n "$DOXYGEN"])
 
 dnl Check for Python