Require bison 2.6 or later.

It turns out the apiprefix define is quite new (I didn't realize it at the
time) and I'd rather get FTBFS reports based on configure catching the
error than gcc catching it.

INSTALL

@@ -23,7 +23,7 @@ The following packages are required to build QuakeForge:
 \li GNU autoconf 2.50 or later
 \li GNU automake 1.6 or later
 \li GNU libtool 1.4 or later
-\li GNU bison
+\li GNU bison 2.6 or later
 \li GNU flex
 \li GNU make (BSD make doesn't like some constructs used)
 \li C compiler and preprocessor (gcc and cpp recommended)

config.d/programs.m4

@@ -19,6 +19,18 @@ else
 		AC_MSG_ERROR(GNU bison is required but was not found)
 	fi
 fi
+BISON_REQ=2.6
+AC_MSG_CHECKING([is bison version >= ${BISON_REQ}])
+BISON_VER=`$YACC --version | sed -n '1s/^.*) //p'`
+AS_VERSION_COMPARE([$BISON_VER], [2.6],
+	[AC_MSG_RESULT([no])
+	LOC=`which ${YACC}`
+	AC_MSG_ERROR(
+		[GNU bison >= 2.6 is required. $BISON_VER found in $LOC]
+	)],
+	AC_MSG_RESULT([yes]),
+	AC_MSG_RESULT([yes])
+)
 
 AM_PROG_LEX
 if echo $LEX | grep -v flex > /dev/null; then