Processing to fix incompatible -O and gcc flags

[jonahbeckford]

Main changes:

1. Accept BUILD_TINY_HTTPD_OPTLEVEL envvar to adjust the -O level. Defaults to 2. Can be negative to remove it entirely, which fixes errors with MSVC which will bail on incompatible options.
2. Do not use -fPIC with MSVC

---

Motivation

This is just a warning for most people:

tiny_httpd_ws_stubs.c
cl : Command line warning D9002 : ignoring unknown option '-std=c99'
cl : Command line warning D9002 : ignoring unknown option '-fPIC'

But if you compile with debug flags like I do, you can get:

File "DkFilesD/_opam/.opam-switch/build/tiny_httpd.0.17.0/src/ws/dune", line 10, characters 11-30:
10 |     (names tiny_httpd_ws_stubs)
                ^^^^^^^^^^^^^^^^^^^
          cl DkFilesD/_opam/.opam-switch/build/tiny_httpd.0.17.0/src/ws/tiny_httpd_ws_stubs.obj (exit 2)
(cd _build/default/DkFilesD/_opam/.opam-switch/build/tiny_httpd.0.17.0/src/ws && Y:/VS/VC/Tools/MSVC/14.38.33130/bin/Hostx64/x64/cl.exe -nologo -Gy- -MDd -DWIN32 -D_WINDOWS -Ob0 -Od -RTC1 -D_CRT_SECURE_NO_DEPRECATE -nologo -Gy- -MDd -DWIN32 -D_WINDOWS -Ob0 -Od -RTC1 -D_CRT_SECURE_NO_DEPRECATE -std=c99 -fPIC -O2 -I Y:/source/dksdk-foundations/build/d/st-d/o/lib/ocaml -I Y:/source/dksdk-foundations/build/d/st-d/o/lib/ocaml\threads -I ../../../hmap.0.8.1+dune/src -I ../../../iostream.0.3/src/core -I ../../../iostream.0.3/src/types -I ../../../seq.base/src -I ../core /Fotiny_httpd_ws_stubs.obj -c tiny_httpd_ws_stubs.c)
cl : Command line warning D9025 : overriding '/Od' with '/O2'
cl : Command line error D8016 : '/RTC1' and '/O2' command-line options are incompatible

All Changes

1. No longer using unknown MSVC options like -fPIC with MSVC.

2. BUILD_TINY_HTTPD_OPTLEVEL can influence and remove -O2.

3. Had to bump up dune to 3.2 or else:

   Error: 'not' is only available since version 3.2 of the dune language. Please
   update your dune-project file to have (lang dune 3.2).
   

Testing

I used a different switch than what is in "Motivation" so it is easy to repeat.

With this PR (scroll to the right):

$ opam exec -- ocamlc -config-var native_c_compiler
cl -nologo -O2 -Gy- -MD    -D_CRT_SECURE_NO_DEPRECATE 

$ opam exec -- dune build -p tiny_httpd --display short
          cl src/ws/tiny_httpd_ws_stubs.obj
(cd _build/default/src/ws && Y:\VS\VC\Tools\MSVC\14.40.33807\bin\HostX64\x64\cl.exe -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE -O2 -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml\threads -I Y:\source\MlFront_Archive\_opam\lib\hmap -I Y:\source\MlFront_Archive\_opam\lib\iostream -I Y:\source\MlFront_Archive\_opam\lib\logs -I Y:\source\MlFront_Archive\_opam\lib\seq -I ../core /Fotiny_httpd_ws_stubs.obj -c tiny_httpd_ws_stubs.c)

$ opam exec -- env BUILD_TINY_HTTPD_OPTLEVEL=1 dune build -p tiny_httpd --display short
          cl src/ws/tiny_httpd_ws_stubs.obj
(cd _build/default/src/ws && Y:\VS\VC\Tools\MSVC\14.40.33807\bin\HostX64\x64\cl.exe -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE -O1 -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml\threads -I Y:\source\MlFront_Archive\_opam\lib\hmap -I Y:\source\MlFront_Archive\_opam\lib\iostream -I Y:\source\MlFront_Archive\_opam\lib\logs -I Y:\source\MlFront_Archive\_opam\lib\seq -I ../core /Fotiny_httpd_ws_stubs.obj -c tiny_httpd_ws_stubs.c)
cl : Command line warning D9025 : overriding '/O2' with '/O1'

$ opam exec -- env BUILD_TINY_HTTPD_OPTLEVEL=0 dune build -p tiny_httpd --display short
          cl src/ws/tiny_httpd_ws_stubs.obj
(cd _build/default/src/ws && Y:\VS\VC\Tools\MSVC\14.40.33807\bin\HostX64\x64\cl.exe -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE -O0 -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml\threads -I Y:\source\MlFront_Archive\_opam\lib\hmap -I Y:\source\MlFront_Archive\_opam\lib\iostream -I Y:\source\MlFront_Archive\_opam\lib\logs -I Y:\source\MlFront_Archive\_opam\lib\seq -I ../core /Fotiny_httpd_ws_stubs.obj -c tiny_httpd_ws_stubs.c)
cl : Command line warning D9002 : ignoring unknown option '-O0'

$ opam exec -- env BUILD_TINY_HTTPD_OPTLEVEL=-1 dune build -p tiny_httpd --display short
          cl src/ws/tiny_httpd_ws_stubs.obj
(cd _build/default/src/ws && Y:\VS\VC\Tools\MSVC\14.40.33807\bin\HostX64\x64\cl.exe -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml -I C:/Users/beckf/AppData/Local/Programs/DKMLNA~1/lib/ocaml\threads -I Y:\source\MlFront_Archive\_opam\lib\hmap -I Y:\source\MlFront_Archive\_opam\lib\iostream -I Y:\source\MlFront_Archive\_opam\lib\logs -I Y:\source\MlFront_Archive\_opam\lib\seq -I ../core /Fotiny_httpd_ws_stubs.obj -c tiny_httpd_ws_stubs.c)

[c-cube]

Very nice, thanks! Took me a second to get the rules but it's nicely done :)