Build without examples fails on Gentoo with "error: ‘syscall’ was not declared in this scope" (multiple times)

[slaufmann]

After the default build failed I tried building fastflow without the examples as described in the Build.me file.
But the make command failed with the following errors:

[ 12%] Building CXX object tests/CMakeFiles/perf_test_noalloc.dir/perf_test_noalloc.cpp.o
In file included from /home/students/slaufmann/Software/mc-fastflow/tests/perf_test_noalloc.cpp:34:0:
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp: In function ‘long int ff_getThreadID()’:
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp:37:37: error: ‘syscall’ was not declared in this scope
 #define gettid() syscall(__NR_gettid)
                                     ^
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp:57:13: note: in expansion of macro ‘gettid’
     return  gettid();
             ^~~~~~
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp: In function ‘int ff_setPriority(int)’:
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp:37:37: error: ‘syscall’ was not declared in this scope
 #define gettid() syscall(__NR_gettid)
                                     ^
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp:123:39: note: in expansion of macro ‘gettid’
         if (setpriority(PRIO_PROCESS, gettid(),priority_level) != 0) {
                                       ^~~~~~
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp: In function ‘int ff_mapThreadToCpu(int, int)’:
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp:37:37: error: ‘syscall’ was not declared in this scope
 #define gettid() syscall(__NR_gettid)
                                     ^
/home/students/slaufmann/Software/mc-fastflow/ff/mapping_utils.hpp:174:27: note: in expansion of macro ‘gettid’
     if (sched_setaffinity(gettid(), sizeof(mask), &mask) != 0)
                           ^~~~~~
make[2]: *** [tests/CMakeFiles/perf_test_noalloc.dir/build.make:63: tests/CMakeFiles/perf_test_noalloc.dir/perf_test_noalloc.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:281: tests/CMakeFiles/perf_test_noalloc.dir/all] Error 2
make: *** [Makefile:139: all] Error 2

My kernel version and system information as of uname -a is:
4.9.16-gentoo #1 SMP PREEMPT Fri Apr 21 13:14:04 CEST 2017 x86_64 Intel(R) Core(TM) i5-6260U CPU @ 1.80GHz GenuineIntel GNU/Linux

A full log of the terminal output can be found on this gist: https://gist.github.com/slaufmann/d8ddc91d9897fb8c629adecca6b4bd00#file-buildloggentoo_2

If I could help with providing any additional information please let me know.

Thanks.