assert: add a partial assert implementation (LDEPS)

This currently does not link because fprintf, stderr and abort are
missing on most platforms. But the code is there.

CMakeLists.txt

@@ -58,6 +58,7 @@ endif()
 # Building
 
 set(SOURCES
+  src/libc/assert/assert.c
   src/libc/stdio/vsnprintf.c
   src/libc/stdio/sprintf.c
   src/libc/stdio/dprintf.c

include/assert.h

@@ -0,0 +1,27 @@
+/*
+** assert.h> can be included several times and the assert macro is redefined
+** every time to match the definition of NDEBUG (7.2§1)
+*/
+#ifdef __ASSERT_H__
+# undef __ASSERT_H__
+# undef assert
+#endif
+
+#define __ASSERT_H__
+
+/* Internal function that prints error message and aborts */
+void __assert_fail(char const *__file, int __line, char const *__function,
+	char const *__expression);
+
+#ifdef NDEBUG
+# define assert(ignore) ((void)0)
+#else
+# define assert(expression) \
+	((expression) \
+	? (void)0 : \
+	: __assert_fail(__FILE__, __LINE__, __PRETTY_FUNCTION__, #expression))
+#endif
+
+#ifndef __cplusplus
+# define static_assert _Static_assert
+#endif

src/libc/assert/assert.c

@@ -0,0 +1,11 @@
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void __assert_fail(char const *file, int line, char const *func, char const
+	*expression)
+{
+	fprintf(stderr, "%s:%d:%s: assertion failed: %s\n",
+		file, line, func, expression);
+	abort();
+}