Fix detection of xmmsctrl.h

Fix detection of xmmsctrl.h

checking xmmsctrl.h usability... yes
checking xmmsctrl.h presence... no
configure: WARNING: xmmsctrl.h: accepted by the compiler, rejected by the preprocessor!
configure: WARNING: xmmsctrl.h: proceeding with the compiler's result
checking for xmmsctrl.h... yes
checking for xmms_remote_get_info in -lxmms... yes

AC_CHECK_HEADER uses the preprocessor, which doesn't look at CFLAGS.
Use CPPFLAGS so it uses the right include path to look for xmmsctrl.h