Link fails on CentOS6

[fadushin]

The linker is unable to resolve clock-gettime with the default linker flags on CentOS 6.

12:12:02 marx:/work/src/github/fadushin/AtomVM/build> echo ${CC}
clang
12:15:37 marx:/work/src/github/fadushin/AtomVM/build> clang --version
clang version 3.4.2 (tags/RELEASE_34/dot2-final)
Target: x86_64-redhat-linux-gnu
Thread model: posix
12:15:47 marx:/work/src/github/fadushin/AtomVM/build> make
[ 1%] Built target generated-nifs-hash
[ 1%] Built target generated
[ 11%] Built target libAtomVM
Linking C executable AtomVM
CMakeFiles/AtomVM.dir/platforms/Linux/sys.c.o: In function sys_waitevents': /work/src/github/fadushin/AtomVM/src/platforms/Linux/sys.c:(.text+0x1b): undefined reference to clock_gettime'
/work/src/github/fadushin/AtomVM/src/platforms/Linux/sys.c:(.text+0x320): undefined reference to clock_gettime' CMakeFiles/AtomVM.dir/platforms/Linux/sys.c.o: In function sys_set_timestamp_from_relative_to_abs':
/work/src/github/fadushin/AtomVM/src/platforms/Linux/sys.c:(.text+0x43b): undefined reference to clock_gettime' CMakeFiles/AtomVM.dir/platforms/Linux/sys.c.o: In function sys_time':
/work/src/github/fadushin/AtomVM/src/platforms/Linux/sys.c:(.text+0x4e8): undefined reference to `clock_gettime'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [src/AtomVM] Error 1
make[1]: *** [src/CMakeFiles/AtomVM.dir/all] Error 2
make: *** [all] Error 2

Adding rt to the end of the linker flags seems to fix this.