[PATCH] configure.ac: Fix autoconf quoting breaking AC_FUNC_MEMCMP

[Sam James]

See the linked autoconf bug for details, but the gist is that AC_FUNC_MEMCMP
ends up lacking the needed includes for string.h if a raw 'if' is used as
opposed to AS_IF.

Quote properly with AS_IF to allow proper generation of the AC_FUNC_MEMCMP
test and avoid -Wimplicit-function-declaration (which will be fatal in some
compilers soon, like Clang 16) in its test.

Bug: https://savannah.gnu.org/support/?110530
---
 configure.ac | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 7121f94..4e0f72f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -35,11 +35,11 @@ AC_ARG_WITH(readline,
        ac_cv_use_readline=yes)
 
 dnl Check for readline library _and_ headers
-if test x"$ac_cv_use_readline" = "xyes"; then
+AS_IF([test x"$ac_cv_use_readline" = "xyes"], [
        AC_CHECK_LIB(ncurses, tputs)
        AC_CHECK_LIB(readline, readline)
        AC_CHECK_HEADERS(readline/readline.h readline/history.h)
-fi
+])
 
 dnl Checks for header files.
 AC_HEADER_STDC
-- 
2.39.1