library: Don't use SIGPWR on FreeBSD

FreeBSD doesn't have SIGPWR so makes no sense in warning and assuming
its 29.

Picked from master, but the file has moved from proc/sig.c to
library/signals.c

References:
 commit 8abd0c92ab
 https://bugs.debian/org/832148

Signed-off-by: Craig Small <csmall@enc.com.au>

lib/signals.c

@@ -56,7 +56,7 @@
 #endif
 
 /* It seems the SPARC libc does not know the kernel supports SIGPWR. */
-#ifndef SIGPWR
+#if defined(__linux__) && !defined(SIGPWR)
 #  warning Your header files lack SIGPWR. (assuming it is number 29)
 #  define SIGPWR 29
 #endif
@@ -84,7 +84,9 @@ static const mapstruct sigtable[] = {
   {"PIPE",   SIGPIPE},
   {"POLL",   SIGPOLL},  /* IO */
   {"PROF",   SIGPROF},
+#ifdef SIGPWR
   {"PWR",    SIGPWR},
+#endif
   {"QUIT",   SIGQUIT},
   {"SEGV",   SIGSEGV},
 #ifdef SIGSTKFLT