- pop_macro incorrect with initially undefined macro
- horrible implementation (tcc_open_bf)
- crashes eventually (abuse of Sym->prev_tok)
- the (unrelated) asm_label part is the opposite of a fix
(Despite of its name this variable has nothing to do with
the built-in assembler)
This reverts commit 0c8447db79.
* give warning if pragma is unknown for tcc
* don't free asm_label in sym_free(),
it's a job of the asm_free_labels().
The above pragmas are used in the mingw headers.
Thise pragmas are implemented in gcc-4.5+ and current
clang.