Try fixing a compilation error when pthread does not exist.

CMakeLists.txt

@@ -458,8 +458,15 @@ if (UNIX)
         endif()
     endif()
     
-    list(APPEND COMMON_LINK_LIBRARIES pthread) # Some compilers will not link to pthread by default so list it here just in case.
-    list(APPEND COMMON_LINK_LIBRARIES m)
+    find_library(LIB_PTHREAD NAMES pthread)
+    if(LIB_PTHREAD)
+        list(APPEND COMMON_LINK_LIBRARIES pthread) # Some compilers will not link to pthread by default so list it here just in case.
+    endif()
+
+    find_library(LIB_M NAMES m)
+    if(LIB_M)
+        list(APPEND COMMON_LINK_LIBRARIES m)
+    endif()
 
     # If we're compiling for 32-bit ARM we need to link to -latomic.
     if(CMAKE_SYSTEM_PROCESSOR MATCHES "^arm" AND NOT CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")