fix(pkg): compat.opencl builds its POSIX loader on macOS; the probe member is unix-only

The macOS block built no sources and a consumer failed at link on
khrIcd_free_getenv. Upstream builds the same dlopen-based sources on Apple
platforms; a runner there reports zero platforms, the loader's own answer.
There is no windows entry, so the member declares its dependency under
cfg(unix) and compiles to a skip elsewhere.

Author: Sunrisepeak

pkgs/c/compat.opencl.lua

@@ -133,10 +133,20 @@ package = {
         },
 
         macosx = {
-            -- Apple ships its own libOpenCL as a framework and there is no ICD
-            -- mechanism, so the loader is built without the linux sources and a
-            -- consumer links the framework instead of dispatching.
-            sources = {},
+            -- The same POSIX sources upstream builds on Apple platforms: the
+            -- loader enumerates `/etc/OpenCL/vendors` and `OCL_ICD_FILENAMES`
+            -- through dlopen there exactly as on Linux. A macOS runner has no
+            -- vendors directory and reports zero platforms, which is the
+            -- loader's own answer and what tests/examples/opencl asserts.
+            -- Apple's OpenCL framework is not involved: a program that links
+            -- this package dispatches through this loader, not the framework.
+            targets = { ["opencl"] = { kind = "shared", soname = "libOpenCL.1.dylib" } },
+            sources = {
+                "*/loader/linux/icd_linux.c",
+                "*/loader/linux/icd_linux_envvars.c",
+                "*/loader/linux/icd_linux_library.c",
+            },
+            ldflags = { "-ldl" },
             runtime = { capabilities = { "opencl.icd.driver" } },
         },
     },

tests/examples/opencl/mcpp.toml

@@ -2,8 +2,14 @@
 name = "opencl-tests"
 version = "0.1.0"
 
-# The loader and the runtime adapter. The adapter is what lets the loader's
-# dlopen of a host ICD resolve from inside mcpp's own C library; on a runner
-# with no driver it is empty, and the loader then reports zero platforms.
-[dependencies.compat]
+# The loader and, on Linux, the runtime adapter it depends on. The adapter is
+# what lets the loader's dlopen of a host ICD resolve from inside mcpp's own
+# C library; on a runner with no driver it is empty and the loader reports
+# zero platforms. compat.opencl has no windows entry: Windows drivers are
+# enumerated through the registry, which this index does not model yet, so
+# the test compiles to a skip there.
+[target.'cfg(unix)'.dependencies.compat]
 opencl = "2026.05.29"
+
+[target.'cfg(unix)'.build]
+cxxflags = ["-DHAVE_OPENCL_LOADER=1"]

tests/examples/opencl/tests/platforms.cpp

@@ -12,10 +12,16 @@
 // `<xmmintrin.h>`, which pulls `<stdlib.h>` into the global module before the
 // std module is imported, and GCC then reports conflicting language linkage
 // for the declarations both provide.
+#include <cstdio>
+#if !defined(HAVE_OPENCL_LOADER)
+int main() {
+    std::printf("compat.opencl: skipped (no windows entry; drivers are enumerated through the registry there)\n");
+    return 0;
+}
+#else
 #define CL_TARGET_OPENCL_VERSION 300
 #include <CL/cl.h>
 #include <CL/cl_ext.h>
-#include <cstdio>
 #include <vector>
 
 int main() {
@@ -51,3 +57,4 @@ int main() {
     }
     return 0;
 }
+#endif