Unknown CMake command "flex_target"

The module does not compile. More precisely

CMake Error at dom/lgm/ngin/CMakeLists.txt:7 (flex_target):
  Unknown CMake command "flex_target".

and

Determining if the pthread_create exist failed with the following output:
Change Dir: /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp

Run Build Command:/usr/bin/make "cmTryCompileExec713796389/fast"
/usr/bin/make -f CMakeFiles/cmTryCompileExec713796389.dir/build.make CMakeFiles/cmTryCompileExec713796389.dir/build
make[1]: Entering directory `/usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp'
/usr/bin/cmake -E cmake_progress_report /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp/CMakeFiles 1
Building C object CMakeFiles/cmTryCompileExec713796389.dir/CheckSymbolExists.c.o
/usr/bin/cc    -o CMakeFiles/cmTryCompileExec713796389.dir/CheckSymbolExists.c.o   -c /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTryCompileExec713796389
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTryCompileExec713796389.dir/link.txt --verbose=1
/usr/bin/cc       CMakeFiles/cmTryCompileExec713796389.dir/CheckSymbolExists.c.o  -o cmTryCompileExec713796389 -rdynamic
CMakeFiles/cmTryCompileExec713796389.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text+0x16): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[1]: *** [cmTryCompileExec713796389] Error 1
make[1]: Leaving directory `/usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp'
make: *** [cmTryCompileExec713796389/fast] Error 2

File /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#include <pthread.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef pthread_create
  return ((int*)(&pthread_create))[argc];
#else
  (void)argc;
  return 0;
#endif
}

Determining if the include file gmpxx.h exists failed with the following output:
Change Dir: /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp

Run Build Command:/usr/bin/make "cmTryCompileExec3968240982/fast"
/usr/bin/make -f CMakeFiles/cmTryCompileExec3968240982.dir/build.make CMakeFiles/cmTryCompileExec3968240982.dir/build
make[1]: Entering directory `/usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp'
/usr/bin/cmake -E cmake_progress_report /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp/CMakeFiles 1
Building CXX object CMakeFiles/cmTryCompileExec3968240982.dir/CheckIncludeFile.cxx.o
/usr/bin/g++-5    -O3 -Wall -funroll-loops -finline-functions -fomit-frame-pointer -ffast-math -mfpmath=sse -msse3 -std=c++14     -o CMakeFiles/cmTryCompileExec3968240982.dir/CheckIncludeFile.cxx.o -c /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp/CheckIncludeFile.cxx
In file included from /usr/include/x86_64-linux-gnu/gmp.h:51:0,
                 from /usr/include/gmpxx.h:33,
                 from /usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp/CheckIncludeFile.cxx:1:
/usr/include/c++/5/cstddef:51:11: error: ‘::max_align_t’ has not been declared
   using ::max_align_t;
           ^
make[1]: *** [CMakeFiles/cmTryCompileExec3968240982.dir/CheckIncludeFile.cxx.o] Error 1
make[1]: Leaving directory `/usr/local_machine/dune-trunk/build/dune-uggrid/CMakeFiles/CMakeTmp'
make: *** [cmTryCompileExec3968240982/fast] Error 2


Determing location of GMP failed:
Include directory: /usr/include
gmp library directory: /usr/lib/x86_64-linux-gnu/libgmp.so
gmpx library directory: /usr/lib/x86_64-linux-gnu/libgmpxx.so

I am using Ubuntu 14.04 LTS, g++5.3 and CMake 2.8.12.2.