Reorganize the source tree.

* Documentation is now in "docs".
 * Source code is now in "src".
 * Misc. fixes here and there so that everything still works.

I think I got everything in this commit, but I only tested this
on Linux (Make) and Windows (CMake), so I might've messed
something up on other platforms...

.gitignore

@@ -5,7 +5,7 @@ a.out
 *.log
 tcc_g
 tcc
-/*-tcc
+/src/*-tcc
 tc2.c
 doc
 tc3s.c
@@ -63,7 +63,7 @@ lib/i386-win32
 lib/arm
 lib/arm64
 tcc-doc.info
-conftest*
+conftest*!conftest.c
 tiny_libmaker
 *.dSYM
 *~