test1148: disable if decimal separator is not point

Modifying the locale with environment variables doesn't work for native
Windows applications. Just disable the test in this case if the decimal
separator is something different than a point. Use a precheck with a
small C program to achieve that.

Closes https://github.com/curl/curl/pull/2786

tests/data/test1148

@@ -37,6 +37,9 @@ progress-bar
  <command>
 http://%HOSTIP:%HTTPPORT/1148 -# --stderr log/stderrlog1148
 </command>
+<precheck>
+perl -e '$ENV{"LC_NUMERIC"} = "en_US.UTF-8"; system("./libtest/chkdecimalpoint") and die("Test requires point as decimal separator");'
+</precheck>
 <setenv>
 LC_ALL=
 LC_NUMERIC=en_US.UTF-8