use -lpthreads instead of -pthreads

https://github.com/google/benchmark/blob/0811f1d782455b3c80285bebf934a7045d845ed3/src/CMakeLists.txt#L42

target_link_libraries(benchmark -lpthreads)

NAG Fortran Compiler had the compile flag -pthread in Version < 6.0.
Using a recent version with cmake ends up having -pthread in the linker command, which results in the error:

nagfor 6.2:
"Option error: Unrecognised option -pthread"

Using -lpthreads instead is working fine.

Closed cmake issue: https://gitlab.kitware.com/cmake/cmake/issues/20103#note_670921