[varnishcache/varnish-cache] b4902b: Use AC_COMPILE_IFELSE

[Federico G. Schwindt]

  Branch: refs/heads/master
  Home:   https://github.com/varnishcache/varnish-cache
  Commit: b4902bfdf779da2d759d9e01ff15628af3659a85
      https://github.com/varnishcache/varnish-cache/commit/b4902bfdf779da2d759d9e01ff15628af3659a85
  Author: Federico G. Schwindt <fgsch at lodoss.net>
  Date:   2015-11-12 (Thu, 12 Nov 2015)

  Changed paths:
    M configure.ac

  Log Message:
  -----------
  Use AC_COMPILE_IFELSE

AC_PREPROC_IFELSE is frowned upon.  Take into account installations
outside the compiler's default search path.