The problem was happening when running cmake. Though, in this case cmake was not the problem the error was silent and the -lpthreads related error/warning was the only thing being written to the cmake error log file, although that was not causing any issue.
I’ve done a minimal version of the cmakelists.txt and started testing it line by line until I found which package was causing it to stop: finally I found it was a version mismatch…
Hint: search for the actual error message
Typically you’d look for the last error message. However, this (often useful) strategy in such cases leads astray.
What you are looking at is the CMakeCache.txt, the CMakeOutput.log or the CMakeError.log. How comes? When some of the macros or tests in the configure phase fails, CMake “helpfully” dumps these files to the output. Unfortunately, these files can be thousands of lines long, and typically contain lots of “*** Error: xyz" entries, for various configure checks. The one for “-lpthreads” just accidentally happened to be the last one in the log…
Solution: go through the log from the top, identify the section with the configure checks, find the last configure check prior to the point, where CMake identifies failure and dumps its logs. You might also try so search for the text “Configuring incomplete, errors occurred!“
Typically you’ll either find a very precise actual error message there, or at least you find the name / path of the macro or function called last, and this allows you to pinpoint down what actually went wrong.