user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/options/glibc/generic/string.cpp

@@ -0,0 +1,32 @@
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE
+#endif
+#include <type_traits>
+#include <string.h>
+
+/* This is a bit of a weird detail of the GNU implementation and C's lack of
+ * overloading and strictness: GNU takes const char * and returns a char * so
+ * that it autocasts to your desired constness, this function never actually
+ * modifies the string.
+ */
+char *__mlibc_gnu_basename_c(const char *path) {
+	char *basename_component = strrchr(path, '/');
+	if (!basename_component) {
+		return const_cast<char *>(path);
+	}
+	return basename_component + 1;
+}
+
+
+/* GNU exposes these overloads, and as a result, we should probably have them
+ * checked, to make sure we actually match expectations.
+ */
+static_assert(
+	std::is_same_v<decltype(basename((const char *)nullptr)), const char*>,
+	"C++ overloads broken"
+);
+
+static_assert(
+	std::is_same_v<decltype(basename((char *)nullptr)), char*>,
+	"C++ overloads broken"
+);