Check if we have CLOCK_MONOTONIC at compile-time

This avoids a subroutine call and also allows us to easily test the code
that doesn't depend on CLOCK_MONOTONIC with
HIJK_TEST_NO_CLOCK_MONOTONIC=1, e.g.:

    HIJK_TEST_NO_CLOCK_MONOTONIC=1 TEST_LIVE=1 prove -Ilib -j 10 -r  t
    HIJK_TEST_NO_CLOCK_MONOTONIC=0 TEST_LIVE=1 prove -Ilib -j 10 -r  t

lib/Hijk.pm

@@ -4,12 +4,17 @@ use warnings;
 use POSIX qw(:errno_h);
 use Socket qw(PF_INET SOCK_STREAM pack_sockaddr_in inet_ntoa $CRLF);
 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
-use Time::HiRes qw(time);
-my $CLOCK = undef;
-eval {
-    Time::HiRes::clock_gettime(Time::HiRes::CLOCK_MONOTONIC());
-    $CLOCK = sub { Time::HiRes::clock_gettime(Time::HiRes::CLOCK_MONOTONIC()) };
-};
+use Time::HiRes qw(time clock_gettime);
+use constant ();
+BEGIN {
+    my $has_CLOCK_MONOTONIC;
+    eval {
+        die "disabled for testing" if $ENV{HIJK_TEST_NO_CLOCK_MONOTONIC};
+        Time::HiRes->import(qw(CLOCK_MONOTONIC));
+        $has_CLOCK_MONOTONIC = 1;
+    };
+    constant->import(HAS_CLOCK_MONOTONIC => $has_CLOCK_MONOTONIC ? 1 : 0);
+}
 
 our $VERSION = "0.13";
 
@@ -30,11 +35,11 @@ sub read_http_message {
     my ($body,$buf,$decapitated,$nbytes,$proto);
     my $status_code = 0;
     my $start = undef;
-    $start = $CLOCK->() if $CLOCK && defined($read_timeout);
+    $start = clock_gettime(CLOCK_MONOTONIC()) if HAS_CLOCK_MONOTONIC && defined($read_timeout);
     vec(my $rin = '', $fd, 1) = 1;
     do {
         my $nfound = select($rin, undef, undef, $read_timeout);
-        $read_timeout -= $CLOCK->() - $start if defined($start);
+        $read_timeout -= clock_gettime(CLOCK_MONOTONIC()) - $start if defined($start);
 
         return (undef,0,undef,undef, Hijk::Error::READ_TIMEOUT)
             if ($nfound != 1 || (defined($read_timeout) && $read_timeout <= 0));
@@ -158,12 +163,11 @@ sub request {
 
     vec(my $rout = '', fileno($soc), 1) = 1;
     my $connect_timeout = selectable_timeout($args->{connect_timeout});
-    my $start = undef;
-    $start = $CLOCK->() if $CLOCK && defined($connect_timeout);
+    my $start; $start = clock_gettime(CLOCK_MONOTONIC()) if HAS_CLOCK_MONOTONIC && defined($connect_timeout);
 
     while ($left > 0) {
         my $nfound = select(undef,$rout, undef, $connect_timeout);
-        $connect_timeout -= $CLOCK->() - $start if defined($start);
+        $connect_timeout -= clock_gettime(CLOCK_MONOTONIC()) - $start if HAS_CLOCK_MONOTONIC and defined($start);
 
         if ($nfound != 1 || (defined($connect_timeout) && $connect_timeout <= 0)) {
             delete $args->{socket_cache}->{$cache_key} if defined $cache_key;