portability: check for qsort_r which does not match POSIX-next definition

FreeBSD (and also Darwin) introduced its own qsort_r, which has different
semantics than the one slated for inclusion in POSIX.  Add a portability
thunk to deal with translating between implementations.

[TT: minor stylistic fixes]

portability/meson.build

@@ -21,6 +21,20 @@ foreach f : check_functions
 endforeach
 
 
+# Check for wrong (non-POSIX) qsort_r prototype
+qsort_r_test = '''
+	#define _GNU_SOURCE
+	#include <stdlib.h>
+	_Static_assert(_Generic((qsort_r),
+		void (*)(void *, size_t, size_t, void *,
+			int (*)(void *, const void *, const void *)) : 1, default: 0),
+		"Bad prototype not matched");
+'''
+if cc.compiles(qsort_r_test, name: 'Test qsort_r non-POSIX prototype')
+	add_project_arguments('-DHAVE_BROKEN_QSORT_R', language: 'c')
+endif
+
+
 if libportability_src.length() > 0
 	libportability = static_library(
 		'portability',

portability/stdlib.h

@@ -1,3 +1,4 @@
+#pragma once
 #include_next <stdlib.h>
 
 #ifdef NEED_QSORT_R
@@ -5,3 +6,28 @@ void qsort_r(void *base, size_t nmemb, size_t size,
 	int (*compar)(const void *, const void *, void *),
 	void *arg);
 #endif
+
+#ifdef HAVE_BROKEN_QSORT_R
+struct __portable_qsort_r_compat_arg {
+	int (*compar)(const void *, const void *, void *);
+	void *arg;
+};
+
+static inline int __portable_qsort_r_compar_compat(void *arg, const void *a, const void *b)
+{
+	struct __portable_qsort_r_compat_arg *compat_arg = arg;
+	return compat_arg->compar(a, b, compat_arg->arg);
+}
+
+static inline void __portable_qsort_r(void *base, size_t nmemb, size_t size,
+				 int (*compar)(const void *, const void *, void *),
+				 void *arg)
+{
+	struct __portable_qsort_r_compat_arg compat_arg = {
+		.compar = compar,
+		.arg = arg,
+	};
+	qsort_r(base, nmemb, size, &compat_arg, __portable_qsort_r_compar_compat);
+}
+#define qsort_r(...) __portable_qsort_r(__VA_ARGS__)
+#endif