Building on musl based linux fails at signals header file: "unknown type name: '__sighandler_t' "
Hey,

trying to build on postmarketOS (Alpine linux based) running on a Pinebook Pro (rk3399). It gets through native_pass 0-3 while waiting on dynarec to compile and fails when done with everything that depends on dynarec at around 30%.

```
[  1%] Building ASM object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/arm64_prolog.S.o
In file included from /home/user/box64/src/dynarec/arm64/dynarec_arm64_functions.c:24:
/home/user/box64/src/include/signals.h:9:11: error: unknown type name '__sighandler_t'
    9 |           __sighandler_t _sa_handler;
      |           ^~~~~~~~~~~~~~
/home/user/box64/src/include/signals.h:19:11: error: unknown type name '__sighandler_t'
   19 |           __sighandler_t _sa_handler;
      |           ^~~~~~~~~~~~~~
...
[ 30%] Built target native_pass3
make: *** [Makefile:166: all] Error 2

```

systeminfo:
```
09:31:47 user@hostname ~/box64/build $ neofetch
                 /\                    user@hostname 
                /  \                   --------- 
               /    \                  OS: postmarketOS v22.06.3 aarch64 
              /      \                 Host: Pine64 Pinebook Pro 
             /        \                Kernel: 6.0.2 
            /          \               Uptime: 18 mins 
            \           \              Packages: 1334 (apk), 35 (flatpak) 
          /\ \____       \             Shell: bash 5.1.16 
         /  \____ \       \            Resolution: 1920x1080 
        /       /  \       \           DE: Plasma 5.24.5 
       /       /    \    ___\          WM: kwin 
      /       /      \  / ____         Theme: Breeze [GTK2/3] 
     /       /        \/ /    \        Icons: breeze-dark [GTK2/3] 
    /       / __________/      \       Terminal: yakuake 
   /        \ \                 \      CPU: (6) @ 1.416GHz 
  /          \ \                 \     Memory: 2740MiB / 3873MiB 
 /           / /                  \
/___________/ /____________________\                           
                                                               

```

[this issue](https://github.com/swoole/swoole-src/issues/721) seem to run into the same problem and fix it by editing the signals header file like so:
```
sed -i "s/struct sigaction {/#ifndef __sighandler_t \ntypedef void (*__sighandler_t)(int);\n#endif\nstruct sigaction\n{/g" ../src/include/signal.h
sed -i "s/union {void (*sa_handler)(int)/__sighandler_t sa_handler/g" ../usr/include/signal.h
```

which just makes my build fail at 2% with the same error:

```
09:48:04 user@hostname ~/box64/build $ make -j$(nproc)
[  1%] Building C object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/arm64_printer.c.o
[  1%] Building C object CMakeFiles/dynarec_native.dir/src/dynarec/dynarec_native.c.o
[  1%] Building C object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/dynarec_arm64_functions.c.o
[  1%] Building C object CMakeFiles/dynarec_native.dir/src/dynarec/dynablock.c.o
[  1%] Building C object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/dynarec_arm64_jmpnext.c.o
[  1%] Generating ../src/wrapped/generated/functions_list.txt
[  1%] Building ASM object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/arm64_prolog.S.o
[  2%] Building ASM object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/arm64_epilog.S.o
In file included from /home/user/box64/src/dynarec/arm64/dynarec_arm64_functions.c:24:
/home/user/box64/src/include/signals.h:9:11: error: unknown type name '__sighandler_t'
    9 |           __sighandler_t _sa_handler;
      |           ^~~~~~~~~~~~~~
/home/user/box64/src/include/signals.h:19:11: error: unknown type name '__sighandler_t'
   19 |           __sighandler_t _sa_handler;
      |           ^~~~~~~~~~~~~~
[  2%] Building ASM object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/arm64_next.S.o
[  2%] Building ASM object CMakeFiles/dynarec_native.dir/src/dynarec/arm64/arm64_lock.S.o
make[2]: *** [CMakeFiles/dynarec_native.dir/build.make:104: CMakeFiles/dynarec_native.dir/src/dynarec/arm64/dynarec_arm64_functions.c.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:123: CMakeFiles/dynarec_native.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
[  2%] Built target WRAPPERS
make: *** [Makefile:166: all] Error 2

```

hmm..
I'll keep digging. 

(I could just put my TwisterOS microSD in and boot from that but would like to make box64 compile on pmOS to try out running some other x86_64 stuff but )