Static function gettid makes troubles when compiling tests,
because the extern declaration exists.
This might not be the best way to fix this, but it makes the tests succeed.

diff -up ck-0.6.0/regressions/common.h.nogettid ck-0.6.0/regressions/common.h
--- ck-0.6.0/regressions/common.h.nogettid	2019-08-21 08:10:51.593209466 +0200
+++ ck-0.6.0/regressions/common.h	2019-08-21 08:11:05.320341728 +0200
@@ -267,13 +267,6 @@ struct affinity {
 #define AFFINITY_INITIALIZER {0, 0}
 
 #ifdef __linux__
-#ifndef gettid
-static pid_t
-gettid(void)
-{
-	return syscall(__NR_gettid);
-}
-#endif /* gettid */
 
 CK_CC_UNUSED static int
 aff_iterate(struct affinity *acb)