feat: add partial support for WASI

This change allows building the `cmark-gfm` target for both `wasm32-unknown-wasi` and `wasm32-unknown-wasip1-threads`. The other targets are not tested.

The differences between two target triples:

- `wasm32-unknown-wasi`
  - C
    - can import `unistd.h`
    - has `__wasi__`
    - does not have `__unix__`
    - **does not have `_REENTRANT`**
    - **does not have `_POSIX_THREADS`**
    - **does not support pthreads API at all**
  - Swift
    - can use `#if os(WASI)`
  - SwiftPM
    - `BuildSettingsCondition`'s `Platform` is `.wasi`
- `wasm32-unknown-wasip1-threads`
  - C
    - can import `unistd.h`
    - has `__wasi__`
    - does not have `__unix__`
    - **has `_REENTRANT` (defined in wasi-libc)**
    - **has `_POSIX_THREADS` (defined in wasi-libc)**
    - **supports a subset of pthreads API**
  - Swift
    - can use `#if os(WASI)`
  - SwiftPM
    - `BuildSettingsCondition`'s `Platform` is `.wasi`

Author: kkebo

Package.swift

@@ -11,12 +11,9 @@ import PackageDescription
     // link time.
     let cSettings: [CSetting] = [
         .define("CMARK_GFM_STATIC_DEFINE", .when(platforms: [.windows])),
-        .define("CMARK_THREADING"),
     ]
 #else
-    let cSettings: [CSetting] = [
-        .define("CMARK_THREADING"),
-    ]
+    let cSettings: [CSetting] = []
 #endif
 
 let package = Package(

api_test/harness.c

@@ -54,7 +54,7 @@ void INT_EQ(test_batch_runner *runner, int got, int expected, const char *msg,
   }
 }
 
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(__wasi__)
 #include <unistd.h>
 
 static char *write_tmp(char const *header, char const *data) {
@@ -79,7 +79,7 @@ void STR_EQ(test_batch_runner *runner, const char *got, const char *expected,
   va_end(ap);
 
   if (!cond) {
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(__wasi__)
     char *got_fn = write_tmp("actual\n", got);
     char *expected_fn = write_tmp("expected\n", expected);
     char buf[1024];

src/include/mutex.h

@@ -3,9 +3,15 @@
 
 #include <stdbool.h>
 
+#ifndef CMARK_THREADING
+#if !defined(__wasi__) || defined(_REENTRANT)
+#define CMARK_THREADING
+#endif
+#endif
+
 #ifdef CMARK_THREADING
 
-#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
+#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__)) || defined(__wasi__)
 #include <unistd.h>
 #endif