configure: Make work2d building abstract from libevent

Makefile.am

@@ -198,7 +198,7 @@ if NEED_DYNCLOCK
 bfgminer_SOURCES += dynclock.c dynclock.h
 endif
 
-if USE_LIBEVENT
+if NEED_BFG_WORK2D
 bfgminer_SOURCES  += work2d.c work2d.h
 endif
 

configure.ac

@@ -127,6 +127,7 @@ need_lowlevel=no
 need_lowl_hid=no
 need_lowl_pci=no
 need_lowl_usb=no
+need_work2d=no
 have_cygwin=false
 have_win32=false
 have_macho=false
@@ -516,6 +517,9 @@ if test "x$libevent" != "xno"; then
 			AC_MSG_WARN([libevent 2.0.3+ not found; stratum proxy will be unavailable])
 		fi
 	])
+	if test "x$libevent" = "xyes"; then
+		need_work2d=yes
+	fi
 fi
 AM_CONDITIONAL([USE_LIBEVENT], [test x$libevent = xyes])
 
@@ -1227,6 +1231,7 @@ AM_CONDITIONAL([NEED_BFG_LOWL_VCOM], [test x$need_lowl_vcom = xyes])
 AM_CONDITIONAL([NEED_BFG_LOWL_HID], [test x$need_lowl_hid = xyes])
 AM_CONDITIONAL([NEED_BFG_LOWL_PCI], [test x$need_lowl_pci = xyes])
 AM_CONDITIONAL([NEED_BFG_LOWLEVEL], [test x$need_lowlevel = xyes])
+AM_CONDITIONAL([NEED_BFG_WORK2D], [test x$need_work2d = xyes])
 AM_CONDITIONAL([HAS_SCRYPT], [test x$scrypt = xyes])
 AM_CONDITIONAL([HAVE_CURSES], [test x$curses = xyes])
 AM_CONDITIONAL([HAVE_SENSORS], [test x$with_sensors = xyes])