Make build hermetic without shell scripts

- Fix some minor issues in ar.com
- Have execve() look for `ape` command
- Rewrite NT paths using /c/ rather /??/c:/
- Replace broken GCC symlinks with .sym files
- Rewrite $PATH environment variables on startup
- Make $(APE_NO_MODIFY_SELF) the default bootloader
- Add all build command dependencies to build/bootstrap
- Get the repository mostly building from source on non-Linux

libc/runtime/finddebugbinary.c

@@ -35,23 +35,24 @@ const char *FindDebugBinary(void) {
   char *p;
   size_t n;
   if (!once) {
-    if (!(res = getenv("COMDBG"))) {
-      p = GetProgramExecutableName();
-      n = strlen(p);
-      if (n > 4 && READ32LE(p + n - 4) == READ32LE(".dbg")) {
-        res = p;
-      } else if (n > 4 && READ32LE(p + n - 4) == READ32LE(".com") &&
-                 n + 4 < ARRAYLEN(buf)) {
-        mempcpy(mempcpy(buf, p, n), ".dbg", 5);
-        if (fileexists(buf)) {
-          res = buf;
-        }
-      } else if (n + 8 < ARRAYLEN(buf)) {
-        mempcpy(mempcpy(buf, p, n), ".com.dbg", 9);
-        if (fileexists(buf)) {
-          res = buf;
-        }
+    p = GetProgramExecutableName();
+    n = strlen(p);
+    if (n > 4 && READ32LE(p + n - 4) == READ32LE(".dbg")) {
+      res = p;
+    } else if (n > 4 && READ32LE(p + n - 4) == READ32LE(".com") &&
+               n + 4 < ARRAYLEN(buf)) {
+      mempcpy(mempcpy(buf, p, n), ".dbg", 5);
+      if (fileexists(buf)) {
+        res = buf;
       }
+    } else if (n + 8 < ARRAYLEN(buf)) {
+      mempcpy(mempcpy(buf, p, n), ".com.dbg", 9);
+      if (fileexists(buf)) {
+        res = buf;
+      }
+    }
+    if (!res) {
+      res = getenv("COMDBG");
     }
     once = true;
   }

libc/runtime/getdosenviron.c

@@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/bits/bits.h"
 #include "libc/runtime/runtime.h"
 #include "libc/str/str.h"
 #include "libc/str/tpenc.h"
@@ -23,6 +24,19 @@
 
 #define ToUpper(c) ((c) >= 'a' && (c) <= 'z' ? (c) - 'a' + 'A' : (c))
 
+forceinline int IsAlpha(int c) {
+  return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z');
+}
+
+forceinline char *MemChr(const char *s, unsigned char c, unsigned long n) {
+  for (; n; --n, ++s) {
+    if ((*s & 255) == c) {
+      return s;
+    }
+  }
+  return 0;
+}
+
 static textwindows noasan noinstrument axdx_t Recode16to8(char *dst,
                                                           size_t dstsize,
                                                           const char16_t *src) {
@@ -45,13 +59,51 @@ static textwindows noasan noinstrument axdx_t Recode16to8(char *dst,
     }
     w = tpenc(x);
     do {
-      if (r.ax + 1 >= dstsize) break;
-      dst[r.ax++] = w;
+      if (r.ax + 1 < dstsize) {
+        dst[r.ax++] = w;
+      } else {
+        break;
+      }
     } while ((w >>= 8));
   }
+  if (r.ax < dstsize) {
+    dst[r.ax] = 0;
+  }
   return r;
 }
 
+textwindows noinstrument noasan void FixPath(char *path) {
+  char *p;
+  size_t i;
+
+  // turn backslash into slash
+  for (p = path; *p; ++p) {
+    if (*p == '\\') {
+      *p = '/';
+    }
+  }
+
+  // turn c:/... into /c/...
+  p = path;
+  if (IsAlpha(p[0]) && p[1] == ':' && p[2] == '/') {
+    p[1] = p[0];
+    p[0] = '/';
+  }
+  for (; *p; ++p) {
+    if (p[0] == ';' && IsAlpha(p[1]) && p[2] == ':' && p[3] == '/') {
+      p[2] = p[1];
+      p[1] = '/';
+    }
+  }
+
+  // turn semicolon into colon
+  for (p = path; *p; ++p) {
+    if (*p == ';') {
+      *p = ':';
+    }
+  }
+}
+
 /**
  * Transcodes NT environment variable block from UTF-16 to UTF-8.
  *
@@ -66,12 +118,17 @@ textwindows noasan noinstrument int GetDosEnviron(const char16_t *env,
                                                   char *buf, size_t size,
                                                   char **envp, size_t max) {
   int i;
+  char *p;
   axdx_t r;
   i = 0;
   --size;
   while (*env) {
     if (i + 1 < max) envp[i++] = buf;
     r = Recode16to8(buf, size, env);
+    if ((p = memchr(buf, '=', r.ax)) && IsAlpha(p[1]) && p[2] == ':' &&
+        (p[3] == '\\' || p[3] == '/')) {
+      FixPath(p + 1);
+    }
     size -= r.ax + 1;
     buf += r.ax + 1;
     env += r.dx;