build(macos/other): librt is not needed for glibc >= 2.17

Since glibc >=2.17 (2012-12-25), librt is a stub-only library.
(https://abi-laboratory.pro/?view=changelog&l=glibc&v=2.17)

* The `clock_*' suite of functions (declared in <time.h>) is now available
  directly in the main C library.  Previously it was necessary to link with
  -lrt to use these functions.  This change has the effect that a
  single-threaded program that uses a function such as `clock_gettime' (and
  is not linked with -lrt) will no longer implicitly load the pthreads
  library at runtime and so will not suffer the overheads associated with
  multi-thread support in other code such as the C++ runtime library.

Current version on Arch Linux is 2.40.

It is however is required for Debian 10, 11 and Ubuntu 20.04.

Signed-off-by: Ivan Prisyazhnyy <john.koepi@gmail.com>

cmake/check.cmake

@@ -24,6 +24,12 @@ if (GALERA_HAVE_SUBUNIT_LIB)
   list(APPEND GALERA_UNIT_TEST_LIBS "${GALERA_HAVE_SUBUNIT_LIB}")
 endif()
 
+# needed on old systems with glibc < 2.17 for timer_create() in libcheck.
+find_library(RT_LIB rt)
+if (RT_LIB)
+  list(APPEND GALERA_UNIT_TEST_LIBS "${RT_LIB}")
+endif()
+
 list(APPEND GALERA_UNIT_TEST_LIBS m)
 list(APPEND GALERA_UNIT_TEST_LIBS ${GALERA_SYSTEM_LIBS})
 

cmake/os.cmake

@@ -5,8 +5,8 @@
 #
 
 find_library(PTHREAD_LIB pthread)
-find_library(RT_LIB rt)
-set(GALERA_SYSTEM_LIBS ${PTHREAD_LIB} ${RT_LIB})
+
+set(GALERA_SYSTEM_LIBS ${PTHREAD_LIB})
 
 if (${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
   # Check if linkage with atomic library is needed for 8 byte atomics

galerautils/tests/CMakeLists.txt

@@ -102,7 +102,7 @@ target_compile_options(deqmap_bench
   PRIVATE
   -Wno-conversion)
 
-target_link_libraries(deqmap_bench galerautilsxx rt)
+target_link_libraries(deqmap_bench galerautilsxx)
 
 #
 # CRC32C micro benchmark.

gcache/src/CMakeLists.txt

@@ -33,7 +33,7 @@ target_link_libraries(gcache galerautilsxx)
 
 add_executable(gcache_test test.cpp)
 
-target_link_libraries(gcache_test gcache pthread rt)
+target_link_libraries(gcache_test gcache pthread)
 
 target_compile_options(gcache_test
   PRIVATE