add prefixnonocaml option

[Et7f3]

ar and gcc can have prefix. So this patch add option to choose prefix for non ocaml tool.

[antoinemine]

Using the CC environment variable, you should already be able to do "CC=x86_64-w64-mingw32-gcc ./configure". I see that there is no support to change ar with an AR environment variable, but we don't seem to use ar at the moment.
Can you confirme whether "CC=prefix-gcc ./configure" would work ? In this case, the patch is not needed.
If not, would adding an AR variable to get "AR=prefix-ar CC=prefix-gcc ./configure" would work ? That would be a preferred solution to adding a -prefixnonocaml option.
It may also be the case that you need to pass the prefixed version of cc to ocaml, which could be done with OCAMLFLAG="-cc prefix-gcc".

[Et7f3]

I have checked: I don't see where ar is used. because of my ar is called /bin/x86_64-w64-mingw32-ar.exe the ./configure script doesn't work.

binary ocaml: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldep: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlmklib: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldoc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ar: not found
required program ar not found
% AR=/bin/x86_64-w64-mingw32-ar.exe ./configure
binary ocaml: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldep: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlmklib: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldoc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ar: not found
required program ar not found```

[Et7f3]

I have commented this line #searchbinreq $ar to test with CC variable.
the result:

as excepted it can't find my gcc because of prefix

% ./configure
binary ocaml: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldep: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlmklib: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldoc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary perl: found in /usr/bin
binary gcc: not found
binary cc: not found
required program cc not found

with variable CC it doesn't fix

% CC=/bin/x86_64-w64-mingw32-gcc.exe ./configure
binary ocaml: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldep: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlmklib: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldoc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary perl: found in /usr/bin
binary /bin/x86_64-w64-mingw32-gcc.exe: found
binary ocamlopt: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
checking compilation with /bin/x86_64-w64-mingw32-gcc.exe : not working
checking compilation with /bin/x86_64-w64-mingw32-gcc.exe -O: not working
cannot compile and link program

try without CC variable: it work fine

% cp /bin/x86_64-w64-mingw32-gcc.exe /bin/cc.exe
% ./configure
binary ocaml: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldep: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamlmklib: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary ocamldoc: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
binary perl: found in /usr/bin
binary gcc: not found
binary cc: found in /usr/bin
binary ocamlopt: found in /cygdrive/c/Users/root/.esy/3_/i/ocaml-4.7.1004-d00f8396/bin
checking compilation with cc -O3 -Wall -Wextra : working
include caml/mlvalues.h: not found
cannot include caml/mlvalues.h

It as a error but is it because of cygwin see my other pull request.

[antoinemine]

Sorry for the delay. OK, if it helps compilation on Cygwin (it doesn't hurt other targets).