Fix compilation flags on Unix-like OSes.

Author: preshing

cmake/BuildSettings.cmake

@@ -4,9 +4,12 @@ if(${MSVC})
     set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} /Zi")
     set(CMAKE_EXE_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS_RELEASE} /debug")
 elseif(${UNIX})
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS_RELEASE} -std=c++11 -pthread")
-    set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS} -g")
-    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS} -g")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+    if(NOT ${CYGWIN})   # Don't specify -pthread on Cygwin
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
+    endif()
+    set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE} -g")
+    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -g")
 endif()
 
 if(${IOS})