Makefile.am: don't ignore user distcheck options
The Automake documentation [1] states that the user could provide DISTCHECK_CONFIGURE_FLAGS, and that AM_DISTCHECK_CONFIGURE_FLAGS should provide additional flags that might be needed. So change DISTCHECK_CONFIGURE_FLAGS for AM_DISTCHECK_CONFIGURE_FLAGS, because as it stands, the user-provided DISTCHECK_CONFIGURE_FLAGS are ignored and overwritten by the Makefile.am.