[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

src/libraries/libc/signal/signal.c

@@ -0,0 +1,58 @@
+/* signal( int, void (*)( int ) )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <signal.h>
+#include <stdlib.h>
+
+void (*_PDCLIB_sigabrt)( int ) = SIG_DFL;
+void (*_PDCLIB_sigfpe)( int )  = SIG_DFL;
+void (*_PDCLIB_sigill)( int )  = SIG_DFL;
+void (*_PDCLIB_sigint)( int )  = SIG_DFL;
+void (*_PDCLIB_sigsegv)( int ) = SIG_DFL;
+void (*_PDCLIB_sigterm)( int ) = SIG_DFL;
+
+void (*signal( int sig, void (*func)( int ) ) )( int )
+{
+    void (*oldhandler)( int );
+    if ( sig <= 0 || func == SIG_ERR )
+    {
+        return SIG_ERR;
+    }
+    switch ( sig )
+    {
+        case SIGABRT:
+            oldhandler = _PDCLIB_sigabrt;
+            _PDCLIB_sigabrt = func;
+            break;
+        case SIGFPE:
+            oldhandler = _PDCLIB_sigfpe;
+            _PDCLIB_sigfpe = func;
+            break;
+        case SIGILL:
+            oldhandler = _PDCLIB_sigill;
+            _PDCLIB_sigill = func;
+            break;
+        case SIGINT:
+            oldhandler = _PDCLIB_sigint;
+            _PDCLIB_sigint = func;
+            break;
+        case SIGSEGV:
+            oldhandler = _PDCLIB_sigsegv;
+            _PDCLIB_sigsegv = func;
+            break;
+        case SIGTERM:
+            oldhandler = _PDCLIB_sigterm;
+            _PDCLIB_sigterm = func;
+            break;
+        default:
+            /* The standard calls for an unspecified "positive value". You
+               will probably want to define a specific value for this.
+            */
+            _PDCLIB_errno = 1;
+            return SIG_ERR;
+    }
+    return oldhandler;
+}