rt: Turn forwards from librt to libc into compat symbols [BZ #24194]

As the  result of commit 6e6249d0b461b952d0f544792372663feb6d792a
("BZ#14743: Move clock_* symbols from librt to libc."), in glibc 2.17,
clock_gettime, clock_getres, clock_settime, clock_getcpuclockid,
clock_nanosleep were added to libc, and the file rt/clock-compat.c
was added with forwarders to the actual implementations in libc.
These forwarders were wrapped in

#if SHLIB_COMPAT (librt, GLIBC_2_2, GLIBC_2_17)

so that they are not present for newer architectures (such as
powerpc64le) with a 2.17 or later ABI baseline.  But the forwarders
were not marked as compatibility symbols.  As a result, on older
architectures, historic configure checks such as

AC_CHECK_LIB(rt, clock_gettime)

still cause linking against librt, even though this is completely
unnecessary.  It also creates a needless porting hazard because
architectures behave differently when it comes to symbol availability.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index e14008d..45792f6 100644
--- a/NEWS
+++ b/NEWS
@@ -14,7 +14,11 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
-  [Add deprecations, removals and changes affecting compatibility here]
+* The functions clock_gettime, clock_getres, clock_settime,
+  clock_getcpuclockid, clock_nanosleep were removed from the librt library
+  for new applications (on architectures which had them).  Instead, the
+  definitions in libc will be used automatically, which have been available
+  since glibc 2.17.
 
 Changes to build and runtime requirements: