libbacktrace: look for _pgmptr on Windows

Patch from Andres Freund:
	* configure.ac: Check for _pgmptr declaration.
	* fileline.c (fileline_initialize): Check for _pgmfptr before
	/proc/self/exec.
	* configure, config.h.in: Regenerate.

configure.ac

@@ -404,6 +404,9 @@ if test "$have_getexecname" = "yes"; then
   AC_DEFINE(HAVE_GETEXECNAME, 1, [Define if getexecname is available.])
 fi
 
+# Check for _pgmptr variable, contains the executable filename on windows
+AC_CHECK_DECLS([_pgmptr])
+
 # Check for sysctl definitions.
 
 AC_CACHE_CHECK([for KERN_PROC],