build: fix compilation on Alpine 3.5

The cbus hang test uses glibc pthread mutex implementation details.
The reason why mutex implementation details is used:
"For the bug reproducing the canceled thread must be canceled
during processing cpipe_flush_cb. We need to synchronize
the main thread and the canceled worker thread for that.
So, thread synchronization has been realized by means of
endpoint's mutex internal field(__data.__lock)."
Therefore, it should not compile in case of using another library.

test/unit/CMakeLists.txt

@@ -104,7 +104,9 @@ target_link_libraries(cbus_stress.test core stat)
 add_executable(cbus.test cbus.c)
 target_link_libraries(cbus.test core unit stat)
 
-if (${CMAKE_HOST_SYSTEM_NAME} MATCHES "Linux")
+include(CheckSymbolExists)
+check_symbol_exists(__GLIBC__ features.h GLIBC_USED)
+if (GLIBC_USED)
     add_executable(cbus_hang.test cbus_hang.c)
     target_link_libraries(cbus_hang.test core unit stat)
 endif ()