configure: replaced more shell constructs with AS_* macros

configure.ac

@@ -1021,14 +1021,17 @@ AS_IF([[test "x$enable_thread_names" = "xno"]],
 
 AM_CONDITIONAL(HAVE_W32, [test "x$os_is_native_w32" = "xyes"])
 w32_shared_lib_exp=no
-if test "x$enable_shared" = "xyes" && test "x$os_is_native_w32" = "xyes"; then
-  if test "x$ac_cv_use_ms_lib_tool" = "xyes" || test -n "$DLLTOOL"; then
-    w32_shared_lib_exp=yes
-  else
-    AC_MSG_WARN([[GNU dlltool or MS lib.exe is required for creating shared library export on W32]])
-    AC_MSG_WARN([[Export library libmicrohttpd.lib will not be created]])
-  fi
-fi
+AS_IF([test "x$enable_shared" = "xyes" && test "x$os_is_native_w32" = "xyes"],
+  [
+    AS_IF([test "x$ac_cv_use_ms_lib_tool" = "xyes" || test -n "$DLLTOOL"],
+      [w32_shared_lib_exp=yes],
+      [
+        AC_MSG_WARN([[GNU dlltool or MS lib.exe is required for creating shared library export on W32]])
+        AC_MSG_WARN([[Export library libmicrohttpd.lib will not be created]])
+      ]
+    )
+  ]
+)
 AM_CONDITIONAL(W32_SHARED_LIB_EXP, [test "x$w32_shared_lib_exp" = "xyes"])
 AM_CONDITIONAL(USE_MS_LIB_TOOL, [test "x$ac_cv_use_ms_lib_tool" = "xyes"])