49634, reported by Claes Nästén: revert 28989

Defining _POSIX_C_SOURCE caused issues on Solaris 10 but doesn't seem
to be needed for musl anymore

ChangeLog

@@ -1,3 +1,9 @@
+2021-12-13  Oliver Kiddle  <opk@zsh.org>
+
+	* 49634, reported by Claes Nästén: configure.ac: revert 28989,
+	defining _POSIX_C_SOURCE caused issues on Solaris 10 but
+	doesn't seem to be needed for musl anymore
+
 2021-12-07  Oliver Kiddle  <opk@zsh.org>
 
 	* 49633: Completion/Unix/Command/_git: update options for git 2.34

configure.ac

@@ -1135,8 +1135,7 @@ dnl Check for sigset_t.  Currently I'm looking in
 dnl <sys/types.h> and <signal.h>.  Others might need
 dnl to be added.
 AC_CACHE_CHECK(for sigset_t, zsh_cv_type_sigset_t,
-[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#define _POSIX_C_SOURCE 200809L
-#include <sys/types.h>
+[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/types.h>
 #include <signal.h>]], [[sigset_t tempsigset;]])],[zsh_cv_type_sigset_t=yes],[zsh_cv_type_sigset_t=no])])
 AH_TEMPLATE([sigset_t],
 [Define to `unsigned int' if <sys/types.h> or <signal.h> doesn't define])