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)