cmake:MSVC/MinGW中导入glog静态库的正确方式

在linux下使用glog静态库，编译连接时只要加上glog静态库文件(.a)就可以了，在windows环境，我也同样的办法在自己的项目中使用glog静态库，用msvc/mingw编译。。。然而链接时报错了，无数的错误都是找不到glog的符号(undefined reference)…
我的项目是cmake生成的工程文件，glog也是我自己用cmake编译出来的，为了查找原因我查看了glog提供的cmake脚本。在lib/cmake/glog/glog-targets.cmake文件中找到如下脚本：

set_target_properties(glog::glog PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "GFLAGS_DLL_DECLARE_FLAG=;GFLAGS_DLL_DEFINE_FLAG=;GLOG_NO_ABBREVIATED_SEVERITIES;GOOGLE_GLOG_DLL_DECL="
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
  INTERFACE_LINK_LIBRARIES "gflags_static"
)

看到这个INTERFACE_COMPILE_DEFINITIONS 属性指定的一堆符号，我明白了。原来在windows下要连接glog静态库，必须要在编译时或在源码中添加下面四个宏定义
GFLAGS_DLL_DECLARE_FLAG，
GFLAGS_DLL_DEFINE_FLAG，
GLOG_NO_ABBREVIATED_SEVERITIES，
GOOGLE_GLOG_DLL_DECL

目的是告诉连接器使用glog静态库的符号表，否则连接器默认会把glog的函数定义当做动态库函数来处理。

如果你是用cmake脚本来编译自己的项目的，如果是用下面的方式来导入glog（caffe就是这么干的,linux下是没问题的）,那么连接时肯定会报错，原因就是没加入上面说的宏定义

# GLOG_ROOT_DIR 是glog 的安装路径
find_path(GLOG_INCLUDE_DIR glog/logging.h
PATHS ${GLOG_ROOT_DIR}/include)
find_library(GLOG_LIBRARY glog 
PATHS ${GLOG_ROOT_DIR}/lib)
...
include_directories(${GLOG_INCLUDE_DIR})
target_link_libraries(your_project ${GLOG_LIBRARY})

在cmake中，加入glog静态库的正确方式是这样的：

# GLOG_ROOT_DIR 是glog 的安装路径
find_package(glog REQUIRED CONFIG HINTS ${GLOG_ROOT_DIR})
set(GLOG_INCLUDE_DIR ${GLOG_ROOT_DIR}/include)
# glog::glog is imported target
set(GLOG_LIBRARY glog::glog)
...
target_link_libraries(your_project ${GLOG_LIBRARY})

上面的代码中使用find_package来查找静态库而不是find_library,find_package（glog）会创建一个名为glog:glog的imported target.
glog::glog就是lib/cmake/glog/glog-targets.cmake定义的imported target(本文最开始提到的),因为这个target的INTERFACE_COMPILE_DEFINITIONS属性中指定了上面4个宏定义，所以聪明的cmake在生成Makefile时会自动在your_project target的编译选项中加入-DGFLAGS_DLL_DECLARE_FLAG -DGFLAGS_DLL_DEFINE_FLAG -DGLOG_NO_ABBREVIATED_SEVERITIES -DGOOGLE_GLOG_DLL_DECL
所以就不需要再执行include_directories(${GLOG_INCLUDE_DIR})

关于imported library, 参见 https://cmake.org/cmake/help/v3.8/manual/cmake-buildsystem.7.html#imported-targets

如下是cmake生成的编译工程文件(flags.make)的样例，可以看到CXX_DEFINES 中已经自动添加了上面的4个宏定义,并且将glog include文件夹也自动加到了CXX_INCLUDES

# CMAKE generated file: DO NOT EDIT!
# Generated by "NMake Makefiles JOM" Generator, CMake Version 3.8

# compile CXX with C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/x86_amd64/cl.exe
CXX_FLAGS = /wd4996 /wd4267 /wd4244 /wd4018 /wd4800 /wd4661 /wd4812 /wd4309 /wd4305 /wd4819  /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MT /O2 /Ob2 /DNDEBUG  

CXX_DEFINES = -DBOOST_ALL_NO_LIB -DCAFFE_VERSION=1.0.0 -DCMAKE_WINDOWS_BUILD -DCPU_ONLY -DGFLAGS_DLL_DECLARE_FLAG="" -DGFLAGS_DLL_DEFINE_FLAG="" -DGFLAGS_IS_A_DLL=0 -DGLOG_NO_ABBREVIATED_SEVERITIES -DGOOGLE_GLOG_DLL_DECL="" -DUSE_LEVELDB -DUSE_LMDB -DUSE_OPENCV

CXX_INCLUDES = -ID:\caffe-static\source\caffe-windows\build.vs2015\include -ID:\caffe-static\release\opencv_windows_vc140_x86_64_mt\include -ID:\caffe-static\release\opencv_windows_vc140_x86_64_mt\include\opencv -ID:\caffe-static\source\caffe-windows\build.vs2015 -ID:\caffe-static\release\boost_windows_vc140_x86_64_mt\include\boost-1_62 -ID:\caffe-static\release\glog_windows_vc140_x86_64_mt\include -ID:\caffe-static\release\gflags_windows_vc140_x86_64_mt\Include -ID:\caffe-static\release\protobuf_windows_vc140_x86_64_mt\include -ID:\caffe-static\release\hdf5_windows_vc140_x86_64_mt\include -ID:\caffe-static\release\lmdb_windows_vc140_x86_64_mt\include -ID:\caffe-static\release\leveldb_windows_vc140_x86_64_mt\include -ID:\caffe-static\release\OpenBLAS_windows_vc140_x86_64_mt\include -ID:\caffe-static\source\caffe-windows\include