将C++exe链接到cmake中的C库

我原以为我很了解cmake,直到我遇到了这个我就是想不出来的问题。我用C语言构建了一个静态库,我正试图用C++对其进行单元测试,但我似乎无法链接到该库中的任何静态函数,我也不知道为什么。我在下面的骨架项目中再现了这个问题:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.6) 
project(myproj) 

add_library(mylib STATIC mylib.c) 

find_package(Boost 1.64.0 COMPONENTS unit_test_framework) 

add_executable(mytestapp mytest.cpp) 
target_include_directories(mytestapp PRIVATE .) 
target_link_libraries(mytestapp mylib) 

enable_testing() 
add_test( mytest mytestapp)

mylib.c:

int add(int a, int b)
{
    return a + b;
}

mylib.h

int add(int a, int b);

#define BOOST_TEST_MODULE mylib_test
#include <boost/test/included/unit_test.hpp>
#include "mylib.h"

BOOST_AUTO_TEST_CASE(mylib_test)
{
    BOOST_TEST( add(2,2) == 4);
}

那么我的输出是:

$ make
Scanning dependencies of target mylib
[ 25%] Building C object CMakeFiles/mylib.dir/mylib.c.o
[ 50%] Linking C static library libmylib.a
[ 50%] Built target mylib
Scanning dependencies of target mytestapp
[ 75%] Building CXX object CMakeFiles/mytestapp.dir/mytest.cpp.o
[100%] Linking CXX executable mytestapp
CMakeFiles/mytestapp.dir/mytest.cpp.o: In function `mylib_test::test_method()':
mytest.cpp:(.text+0x1e411): undefined reference to `add(int, int)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/mytestapp.dir/build.make:96: mytestapp] Error 1
make[1]: *** [CMakeFiles/Makefile2:105: CMakeFiles/mytestapp.dir/all] Error 2
make: *** [Makefile:95: all] Error 2

如果我编译 mylib