Fix 'AC_LINK_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS' (#1139)

Fix 'AC_LINK_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS' (#1139)

... produced during bootstrap.sh by autoconf 2.71

AC_USE_SYSTEM_EXTENSIONS needs to be placed after detecting
which OS and compiler are being used. But before any of the
checks detecting capabilities of those OS and compilers.

AC_CANONICAL_HOST is a prerequisite of the simple_host_os logic.
Attach it to that code block instead of the compiler detection
block.

Replace deprecated '$GCC' variable with autoconf cached
'$ac_cv_c_compiler_gnu' variable used as the real source for the
yes/no value of GCC usability after AC_PROG_CC.