POSIX paths in #include directives don't work with MinGW

[Torbjorn Granlund]

LRN <lrn1986 at gmail.com> writes:

  GMP version: 5.1.1 tarball
  
  How to reproduce: run ./configure in MSYS on W32, using any W32-native
  MinGW toolchain (doesn't matter which one)
  
  What is wrong:
  GMP_INCLUDE_GMP_H and GMP_FUNC_ALLOCA generate things like
  #include "$srcdir/gmp-impl.h"
  $srcdir is expanded to something like
  "/tc/gmp-5.1.1-1/gmp-5.1.1/gmp-h.in", because the shell is POSIXly.
  However, the compiler (MinGW) is a native W32 program, and does not
  understand POSIX paths, fails to compile mp_limb_t test, and configure
  script fails as a result.
  
This is a surprising bug report, given that GMP for MingW is in
widespread use.  I cannot tell why your install is different.

-- 
Torbjörn