v592+ breaks procfs reads

[xpenev]

Files like /proc/cpuinfo cannot be read starting v592

$ echo $(./less --version | sed q; ./less /proc/cpuinfo | wc -c) bytes read
less 609x (POSIX regular expressions) 0 bytes read

In older versions, this used to work

$ echo $(less --version | sed q; less /proc/cpuinfo | wc -c) bytes read
less 590 (PCRE2 regular expressions) 12528 bytes read

The HAVE_PROCFS check added in v592 fails[1] on Linux

AC_TRY_COMPILE([#include <sys/statfs.h>],
  [struct statfs s; s.f_type = PROC_SUPER_MAGIC; (void) fstatfs(0,&s); ],
  [AC_MSG_RESULT(yes); AC_DEFINE(HAVE_PROCFS)], [AC_MSG_RESULT(no)])

[1] Tested on:
Linux Bullseye 5.10.0-18-amd64 #1 SMP Debian 5.10.140-1 (2022-09-02) x86_64 GNU/Linux
Linux Gentoo 5.19.9-gentoo

$ gcc -E -fdirectives-only - <<< '#include <sys/statfs.h>' | grep PROC_SUPER_MAGIC
$ gcc -E -fdirectives-only - <<< '#include <linux/magic.h>' | grep PROC_SUPER_MAGIC
#define PROC_SUPER_MAGIC 0x9fa0

Potential Fix:

diff --git a/ch.c b/ch.c
index bfad09c..c5317ba 100644
--- a/ch.c
+++ b/ch.c
@@ -28,6 +28,9 @@ extern ino_t curr_ino;

 #if HAVE_PROCFS
 #include <sys/statfs.h>
+#if HAVE_LINUX_MAGIC
+#include <linux/magic.h>
+#endif
 #endif

 typedef POSITION BLOCKNUM;
diff --git a/configure.ac b/configure.ac
index f7c6481..f849134 100644
--- a/configure.ac
+++ b/configure.ac
@@ -219,6 +219,8 @@ AH_TEMPLATE([HAVE_CONST],
        [Define HAVE_CONST if your compiler supports the "const" modifier.])
 AH_TEMPLATE([HAVE_STAT_INO],
        [Define HAVE_STAT_INO if your struct stat has st_ino and st_dev.])
+AH_TEMPLATE([HAVE_LINUX_MAGIC],
+       [Define HAVE_LINUX_MAGIC if you have linux/magic.h.])
 AH_TEMPLATE([HAVE_PROCFS],
        [Define HAVE_PROCFS if have have fstatfs with f_type and PROC_SUPER_MAGIC.])
 AH_TEMPLATE([HAVE_TIME_T],
@@ -270,8 +272,15 @@ AC_TRY_COMPILE([#include <sys/types.h>
 #include <sys/stat.h>],
   [struct stat s; dev_t dev = s.st_dev; ino_t ino = s.st_ino;],
   [AC_MSG_RESULT(yes); AC_DEFINE(HAVE_STAT_INO)], [AC_MSG_RESULT(no)])
+AC_MSG_CHECKING(for magic.h)
+AC_TRY_COMPILE([#include <linux/magic.h>], [int magic = PROC_SUPER_MAGIC;],
+  [AC_MSG_RESULT(yes); AC_DEFINE(HAVE_LINUX_MAGIC)], [AC_MSG_RESULT(no)])
 AC_MSG_CHECKING(for procfs)
-AC_TRY_COMPILE([#include <sys/statfs.h>],
+AC_TRY_COMPILE([
+#include <sys/statfs.h>
+#ifdef HAVE_LINUX_MAGIC
+#include <linux/magic.h>
+#endif],
   [struct statfs s; s.f_type = PROC_SUPER_MAGIC; (void) fstatfs(0,&s); ],
   [AC_MSG_RESULT(yes); AC_DEFINE(HAVE_PROCFS)], [AC_MSG_RESULT(no)])

[gwsw]

This should be fixed in 03f011f.