Musl compatibility
__va_copy is a macro and can be detected via #ifdef.
This legacy header appears to be unused. Removing its includes avoids numerous warnings when compiling with musl libc: /usr/include/sys/signal.h:1:2: warning: #warning redirecting incorrect #include <sys/signal.h> to <signal.h> [-Wcpp]
Fixes warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
Avoids aclocal warning