It is recommended that AC_CYGWIN not be used and under MinGW
the previous test was complaining about multiple calls to
AC_CANONICAL_HOST. The new test performs the same function
and also displays the MinGW status if we are compiling on
windows.
Also removed a AX_CCP_IDENT call in the vpi directory and
tgt-null is built by the top configure so there is no need
to try to run configure in that directory.
A native MinGW compile was not setting these to the correct
value. This patch makes them match what Cygwin uses. Adding
the -Wl,--enable-auto-image-base should make vvp load
slightly faster since it loads multiple DLL files.
This configure option causes the installed commands to have
a suffix string that makes them distinct from other versions
that also have a suffix string. This allows for multiple
installed versions of Icarus Verilog.
Also, move installed C/C++ header files into a subdirectory of
their own under the target include directory, to make clearer
the purpose and source of those files.