Add a BeConservative setting to the make system

If it is set, we don't try to use clock_gettime

mk/config.mk.in

@@ -291,6 +291,10 @@ else
 GhcEnableTablesNextToCode=YES
 endif
 
+# When building bindists we set this to yes so that the binaries are as
+# portable as possible.
+BeConservative = NO
+
 #
 # Building various ways?
 # (right now, empty if not).

rts/Makefile

@@ -35,6 +35,10 @@ GhcRtsHcOpts=
 GhcRtsCcOpts=-g -O0
 endif
 
+ifeq "$(BeConservative)" "YES"
+GhcRtsCcOpts += -DBE_CONSERVATIVE
+endif
+
 # -----------------------------------------------------------------------------
 
 # There's nothing for Haddock here...

rts/posix/GetTime.c

@@ -134,7 +134,7 @@ Ticks getThreadCPUTime(void)
     }
     return ((usec * TICKS_PER_SECOND) / 1000000);
 
-#elif defined(HAVE_CLOCK_GETTIME) && defined (_POSIX_THREAD_CPUTIME) && defined(CLOCK_THREAD_CPUTIME_ID) && defined(HAVE_SYSCONF)
+#elif !defined(BE_CONSERVATIVE) && defined(HAVE_CLOCK_GETTIME) && defined (_POSIX_THREAD_CPUTIME) && defined(CLOCK_THREAD_CPUTIME_ID) && defined(HAVE_SYSCONF)
     if (sysconf(_POSIX_THREAD_CPUTIME) != -1) {
         // clock_gettime() gives us per-thread CPU time.  It isn't
         // reliable on Linux, but it's the best we have.