ensure autoconf 2.69 compatiblity in configure.ac

As reported by @jeriedel24 [here](7f6dd75#commitcomment-160100961)
old autoconf does not correctly deal with `test` clauses split across the lines. So we fix it here

Author: dimpase

configure.ac

@@ -310,8 +310,8 @@ fi
    AC_CHECK_HEADER(bzlib.h, [
       AC_SEARCH_LIBS([BZ2_bzCompress], [bz2], [
         AC_PATH_PROG([bzip2_prog], [bzip2])
-        AS_IF([test x$bzip2_prog = x
-	], [AC_MSG_ERROR([could not find bzip2 executable. Cf. https://doc.sagemath.org/html/en/reference/spkg/_prereq.html])])
+        AS_IF([test x$bzip2_prog = x],
+              [AC_MSG_ERROR([could not find bzip2 executable. Cf. https://doc.sagemath.org/html/en/reference/spkg/_prereq.html])])
       ], [AC_MSG_ERROR([could not find libbz2 library. Cf. https://doc.sagemath.org/html/en/reference/spkg/_prereq.html])])
     ], [AC_MSG_ERROR([could not find bzlib.h - the header of libbz2 library. Cf. https://doc.sagemath.org/html/en/reference/spkg/_prereq.html])
    ])