gh-40261: ensure autoconf 2.69 compatiblity in configure.ac
    
As reported by @jeriedel24 [here](https://github.com/sagemath/sage/commi
t/7f6dd7547ecf8c4d23099842f4ad4ace5f2f3072#commitcomment-160100961), old
autoconf does not correctly deal with `test` clauses split across the
lines. So we fix this regression, introduced in #40011,  here.

We still cannot drop autoconf 2.69, as various distros, e.g. Debian, are
very slow in adapting 2.71+

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### ⌛ Dependencies

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URL: #40261
Reported by: Dima Pasechnik
Reviewer(s): Tobias Diez

Author: Release Manager

configure.ac

@@ -314,8 +314,8 @@ 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])
    ])