[GHC] #10861: `configure -C` yields different results on second run

[GHC]

#10861: `configure -C` yields different results on second run
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              Reporter:  thomie      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Build       |           Version:  7.10.2
  System                             |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Autoconf has a feature to cache the results of a configure run (using
 `configure -C`), and use it in subsequent runs. It greatly speeds up those
 subsequent runs.

 * http://www.gnu.org/software/autoconf/manual/autoconf-2.66/html_node
 /Cache-Files.html#Cache-Files
 * http://www.gnu.org/software/autoconf/manual/autoconf-2.66/html_node
 /Caching-Results.html

 At least the `FP_GCC_VERSION` macro in aclocal.m4 is currently buggy. The
 call to AC_CACHE_CHECK should only set the cache variable
 (fp_cv_gcc_version), but it also sets others (GccLT34 and GccLT44) (side
 question: how do those interact with clang?). This means that subsequent
 runs of `configure -C` won't use the right values for GccLT34 and GccLT44,
 when your gcc is actually older than those.

 There are likely other bugs.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10861>
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