[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/j6libc/strtox_prelim.c

@@ -0,0 +1,50 @@
+/* _PDCLIB_strtox_prelim( const char *, char *, int * )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <ctype.h>
+#include <stddef.h>
+#include <string.h>
+
+const char * _PDCLIB_strtox_prelim( const char * p, char * sign, int * base )
+{
+    /* skipping leading whitespace */
+    while ( isspace( *p ) ) ++p;
+    /* determining / skipping sign */
+    if ( *p != '+' && *p != '-' ) *sign = '+';
+    else *sign = *(p++);
+    /* determining base */
+    if ( *p == '0' )
+    {
+        ++p;
+        if ( ( *base == 0 || *base == 16 ) && ( *p == 'x' || *p == 'X' ) )
+        {
+            *base = 16;
+            ++p;
+            /* catching a border case here: "0x" followed by a non-digit should
+               be parsed as the unprefixed zero.
+               We have to "rewind" the parsing; having the base set to 16 if it
+               was zero previously does not hurt, as the result is zero anyway.
+            */
+            if ( memchr( _PDCLIB_digits, tolower(*p), *base ) == NULL )
+            {
+                p -= 2;
+            }
+        }
+        else if ( *base == 0 )
+        {
+            *base = 8;
+        }
+        else
+        {
+            --p;
+        }
+    }
+    else if ( ! *base )
+    {
+        *base = 10;
+    }
+    return ( ( *base >= 2 ) && ( *base <= 36 ) ) ? p : NULL;
+}