Safety belt on -lpthread and -lc

[bapt]

Summary

Ensure that if -lc and -lpthread are found in the LDADD chain they are always
The to last components of the link chain in the following order:
-lpthread then -lc

[kib]

I think this is fine, but please take it with a grain of salt due to my ignorance WRT both build system and make.

Also note that this is good safety belts, but not a complete solution. E.g., it does not help with the problem in r265003, since the buggy order is realized not due to the wrong appearance of -lpthread on the command line, but due to the libraries which required libthr.

[bapt]

Committed