Processing to fix incompatible -O and gcc flags

Two changes:
1. Accept BUILD_TINY_HTTPD_OPTLEVEL envvar to adjust the -O<num> 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

dune-project

@@ -1,4 +1,4 @@
-(lang dune 2.9)
+(lang dune 3.2)
 (name tiny_httpd)
 (generate_opam_files true)
 

src/ws/dune

@@ -1,3 +1,28 @@
+; Set BUILD_TINY_HTTPD_OPTLEVEL to the -O<num> level.
+; Defaults to 2, which means -O2 is the default C optimization flag.
+; Use -1 to remove the -O<num> flag entirely.
+(rule
+ (enabled_if (>= %{env:BUILD_TINY_HTTPD_OPTLEVEL=2} 0))
+ (target optlevel.string)
+ (deps (env_var BUILD_TINY_HTTPD_OPTLEVEL))
+ (action (with-stdout-to %{target} (echo "-O%{env:BUILD_TINY_HTTPD_OPTLEVEL=2}"))))
+(rule
+ (enabled_if (< %{env:BUILD_TINY_HTTPD_OPTLEVEL=2} 0))
+ (target optlevel.string)
+ (deps (env_var BUILD_TINY_HTTPD_OPTLEVEL))
+ (action (with-stdout-to %{target} (echo ""))))
+
+; All compilers will include the optimization level.
+; Non-MSVC compilers will include `-std=c99 -fPIC`.
+(rule
+ (enabled_if (= %{ocaml-config:ccomp_type} msvc))
+ (target cflags.sexp)
+ (action (with-stdout-to %{target} (echo "(%{read:optlevel.string})"))))
+(rule
+ (enabled_if (not (= %{ocaml-config:ccomp_type} msvc)))
+ (target cflags.sexp)
+ (action (with-stdout-to %{target} (echo "(-std=c99 -fPIC %{read:optlevel.string})"))))
+
 (library
  (name tiny_httpd_ws)
  (public_name tiny_httpd.ws)
@@ -7,7 +32,7 @@
  (foreign_stubs
   (language c)
   (names tiny_httpd_ws_stubs)
-  (flags :standard -std=c99 -fPIC -O2))
+  (flags :standard (:include cflags.sexp)))
  (libraries
   (re_export tiny_httpd.core)
   threads))

tiny_httpd.opam

@@ -11,7 +11,7 @@ tags: [
 homepage: "https://github.com/c-cube/tiny_httpd/"
 bug-reports: "https://github.com/c-cube/tiny_httpd/issues"
 depends: [
-  "dune" {>= "2.9"}
+  "dune" {>= "3.2"}
   "seq"
   "base-threads"
   "result"
@@ -38,11 +38,9 @@ build: [
     name
     "-j"
     jobs
-    "--promote-install-files=false"
     "@install"
     "@runtest" {with-test}
     "@doc" {with-doc}
   ]
-  ["dune" "install" "-p" name "--create-install-files" name]
 ]
 dev-repo: "git+https://github.com/c-cube/tiny_httpd.git"

tiny_httpd_camlzip.opam

@@ -8,7 +8,7 @@ license: "MIT"
 homepage: "https://github.com/c-cube/tiny_httpd/"
 bug-reports: "https://github.com/c-cube/tiny_httpd/issues"
 depends: [
-  "dune" {>= "2.9"}
+  "dune" {>= "3.2"}
   "tiny_httpd" {= version}
   "camlzip" {>= "1.06"}
   "iostream-camlzip" {>= "0.2.1"}
@@ -24,11 +24,9 @@ build: [
     name
     "-j"
     jobs
-    "--promote-install-files=false"
     "@install"
     "@runtest" {with-test}
     "@doc" {with-doc}
   ]
-  ["dune" "install" "-p" name "--create-install-files" name]
 ]
 dev-repo: "git+https://github.com/c-cube/tiny_httpd.git"