build: fix unit tests build with -lrt on CentOS 6

After the commit 77fa45bd
('lua: add fiber.top() listing fiber cpu consumption')
the unit tests builds failed like:

/opt/rh/devtoolset-6/root/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/ld:
  ../../src/lib/core/libcore.a(fiber.c.o): undefined reference to symbol
  'clock_gettime@@GLIBC_2.2.5'
//lib64/librt.so.1: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
test/unit/CMakeFiles/cbus.test.dir/build.make:108: recipe for target
  'test/unit/cbus.test' failed
make[2]: *** [test/unit/cbus.test] Error 1

Found that fiber.cc is using now clock_gettime(), which requires -lrt
with glibc. To fix it added librt dependency for core library for glibc.
Due to glibc requires for -lrt for clock_gettime() only for some
versions, check 'man clock_gettime.2':
  'Link with -lrt (only for glibc versions before 2.17).'
the check whether is able to use clock_gettime() w/o librt library is
added.

Close #4639

(cherry picked from commit 99b0ef771135eab66ef3758371b1b5431e21cbff)

CMakeLists.txt

@@ -116,6 +116,12 @@ else ()
     set(HAVE_CLOCK_GETTIME ${HAVE_CLOCK_GETTIME_DECL})
 endif ()
 set(CMAKE_REQUIRED_LIBRARIES "")
+# According to `man 2 clockgettime` the glibc wrapper requires
+# -lrt on glibc versions before 2.17. We need to check whether
+# the function is available without -lrt to set this linker option
+# conditionally.
+check_function_exists(clock_gettime HAVE_CLOCK_GETTIME_WITHOUT_RT)
+
 check_symbol_exists(__get_cpuid cpuid.h HAVE_CPUID)
 
 # Checks for libev

src/lib/core/CMakeLists.txt

@@ -46,3 +46,11 @@ target_link_libraries(core salad small uri decNumber ${LIBEV_LIBRARIES}
 if (ENABLE_BACKTRACE AND NOT TARGET_OS_DARWIN)
     target_link_libraries(core gcc_s ${UNWIND_LIBRARIES})
 endif()
+
+# Since fiber.top() introduction, fiber.cc, which is part of core
+# library, depends on clock_gettime() syscall, so we should set
+# -lrt when it is appropriate. See a comment for
+# HAVE_CLOCK_GETTIME_WITHOUT_RT in ${REPO}/CMakeLists.txt.
+if ("${HAVE_CLOCK_GETTIME}" AND NOT "${HAVE_CLOCK_GETTIME_WITHOUT_RT}")
+    target_link_libraries(core rt)
+endif()