build-sys: discover the path to kexec during build time

https://bugs.freedesktop.org/show_bug.cgi?id=55248

Makefile.am

@@ -165,6 +165,7 @@ AM_CPPFLAGS = \
 	-DUDEVLIBEXECDIR=\"$(udevlibexecdir)\" \
 	-DPOLKIT_AGENT_BINARY_PATH=\"$(bindir)/pkttyagent\" \
 	-DQUOTACHECK=\"$(QUOTACHECK)\" \
+	-DKEXEC=\"$(KEXEC)\" \
 	-I $(top_srcdir)/src \
 	-I $(top_srcdir)/src/shared \
 	-I $(top_srcdir)/src/login \

configure.ac

@@ -76,6 +76,8 @@ AC_PATH_PROG([KILL], [kill], [/usr/bin/kill])
 
 AC_PATH_PROG([KMOD], [kmod], [/usr/bin/kmod])
 
+AC_PATH_PROG([KEXEC], [kexec], [/usr/sbin/kexec])
+
 # gtkdocize greps for '^GTK_DOC_CHECK', so it needs to be on its own line
 m4_ifdef([GTK_DOC_CHECK], [
 GTK_DOC_CHECK([1.18],[--flavour no-tmpl])],

src/core/shutdown.c

@@ -318,7 +318,7 @@ int main(int argc, char *argv[]) {
                                 log_warning("kexec failed. Falling back to normal reboot.");
                         } else {
                                 /* Child */
-                                const char *args[3] = { "/sbin/kexec", "-e", NULL };
+                                const char *args[3] = { KEXEC, "-e", NULL };
                                 execv(args[0], (char * const *) args);
                                 return EXIT_FAILURE;
                         }

src/login/logind-action.c

@@ -81,7 +81,7 @@ int manager_handle_action(
         else if (handle == HANDLE_HYBRID_SLEEP)
                 supported = can_sleep("hybrid-sleep") > 0;
         else if (handle == HANDLE_KEXEC)
-                supported = access("/sbin/kexec", X_OK) >= 0;
+                supported = access(KEXEC, X_OK) >= 0;
         else
                 supported = true;