Check for waitstatus orientation via cmake

Yeah we need the long way around because old glibc versions have weird WEXITSTATUS.

cmake/ConfigureChecks.cmake

@@ -268,6 +268,16 @@ IF (NOT LIBATOMIC_NOT_NEEDED)
     set(ATOMIC_LIBRARY "atomic")
 endif()
 
+check_cxx_source_compiles("
+#include <sys/wait.h>
+
+int main() {
+    static_assert(WEXITSTATUS(0x007f) == 0x7f);
+    return 0;
+}
+"
+HAVE_WAITSTATUS_SIGNAL_RET)
+
 IF (APPLE)
     # Check if mbrtowc implementation attempts to encode invalid UTF-8 sequences
     # Known culprits: at least some versions of macOS (confirmed Snow Leopard and Yosemite)

config_cmake.h.in

@@ -103,6 +103,9 @@
 /* Define to 1 if you have the `wcstod_l' function. */
 #cmakedefine HAVE_WCSTOD_L 1
 
+/* Define to 1 if the status that wait returns and WEXITSTATUS expects is signal and then ret instead of the other way around. */
+#cmakedefine HAVE_WAITSTATUS_SIGNAL_RET 1
+
 /* Define to 1 if the winsize struct and TIOCGWINSZ macro exist */
 #cmakedefine HAVE_WINSIZE 1
 

src/proc.h

@@ -73,7 +73,7 @@ class proc_status_t {
     static constexpr int w_exitcode(int ret, int sig) {
 #ifdef W_EXITCODE
         return W_EXITCODE(ret, sig);
-#elif WEXITSTATUS(0x007f) == 0x7f
+#elif HAVE_WAITSTATUS_SIGNAL_RET
         // It's encoded signal and then status
         // The return status is in the lower byte.
         return ((sig) << 8 | (ret));