Define OMIT_UTMP if there's no utmpx.h.

Without this, the build of e.g. psusan would fail on systems without
that header (such as Termux on Android).

This is similar to how things were pre-cmake, but not identical. We used
to treat lack of updwtmpx() as a reason to OMIT_UTMP (as of f0dfa73982),
but usage of that function got conditionalised in c19e7215dd, so I
haven't restored that exclusion.

cmake/cmake.h.in

@@ -16,6 +16,7 @@
 #cmakedefine01 HAVE_DWMAPI_H
 
 #cmakedefine NOT_X_WINDOWS
+#cmakedefine OMIT_UTMP
 
 #cmakedefine01 HAVE_ASM_HWCAP_H
 #cmakedefine01 HAVE_SYS_AUXV_H

cmake/platforms/unix.cmake

@@ -19,6 +19,7 @@ check_include_file(sys/sysctl.h HAVE_SYS_SYSCTL_H)
 check_include_file(sys/types.h HAVE_SYS_TYPES_H)
 check_include_file(glob.h HAVE_GLOB_H)
 check_include_file(utmp.h HAVE_UTMP_H)
+check_include_file(utmpx.h HAVE_UTMPX_H)
 
 check_symbol_exists(futimes "sys/time.h" HAVE_FUTIMES)
 check_symbol_exists(getaddrinfo "sys/types.h;sys/socket.h;netdb.h"
@@ -56,6 +57,12 @@ else()
   set(NO_IPV6 ON)
 endif()
 
+if(HAVE_UTMPX_H)
+  set(OMIT_UTMP OFF)
+else()
+  set(OMIT_UTMP ON)
+endif()
+
 include(cmake/gtk.cmake)
 
 # See if we have X11 available. This requires libX11 itself, and also

unix/pty.c

@@ -227,6 +227,8 @@ static void setup_utmp(char *ttyname, char *location)
     endutxent();
 
 #if HAVE_UPDWTMPX
+    /* Reportedly, AIX 5.1 has <utmpx.h> and pututxline(), but no
+     * updwtmpx(). */
     updwtmpx(WTMPX_FILE, &utmp_entry);
 #endif