libarchive: Drop use of pthread.h for CMake build

CMake does not use threads so we do not need this in our libarchive.

Utilities/cmlibarchive/CMakeLists.txt

@@ -94,6 +94,8 @@ IF(WIN32)
   SET(_WIN32_WINNT ${WINVER})
 ENDIF(WIN32)
 
+set(HAVE_PTHREAD_H 0) # no threads in CMake
+
 IF("${CMAKE_C_PLATFORM_ID}" MATCHES "^(HP-UX)$")
   ADD_DEFINITIONS(-D_XOPEN_SOURCE=500) # Ask wchar.h for mbstate_t
 ENDIF()