Wrong coverage data for unit tests of a library

I want to test a complex shared library (cxsc) with the help of cmake. After a lot of trial and error i managed to create a html coverage report but the lines I test, with the boost testing framework, are still red.

This is the script I use to create the coverage report:

rm -fr build_coverage
mkdir build_coverage
cd build_coverage
cmake \
    -DCMAKE_BUILD_TYPE=Coverage \
    ..
make
cd ../tests
make clean
make
cd ../build_coverage
make cxsc_coverage

The cmake part where the coverage report gets created:

# Cleanup lcov
COMMAND ${LCOV_PATH} --zerocounters --directory ${CMAKE_CURRENT_SOURCE_DIR}/build_coverage/CMakeFiles/cxsc.dir/src
COMMAND ${LCOV_PATH} --capture --initial --no-external --directory ${CMAKE_CURRENT_SOURCE_DIR}/build_coverage/CMakeFiles/cxsc.dir/src --output-file ${_outputname}.before

# Run tests
COMMAND LD_LIBRARY_PATH=${CMAKE_CURRENT_SOURCE_DIR}/build_coverage ${_testrunner} ${ARGV3}

# Capturing lcov counters and generating report
COMMAND ${LCOV_PATH} --capture --no-checksum --no-external --directory ${CMAKE_CURRENT_SOURCE_DIR}/build_coverage/CMakeFiles/cxsc.dir/src --output-file ${_outputname}.after
COMMAND ${LCOV_PATH} --add-tracefile ${_outputname}.before --add-tracefile ${_outputname}.after --output-file ${_outputname}.info

COMMAND ${GENHTML_PATH} -o ${_outputname} ${_outputname}.info

Makefile of the test binary (I think this is where the error is):

g++ \
    -o test_runner \
    main.cpp \
    test_interval.cpp \
    -I./../src \
    -I./../src/rts \
    -I./../src/asm \
    -I./../src/fi_lib \
    -I./../build_coverage \
    -L./../build_coverage \
    -lcxsc \
    -Wall -Winline \
    -lboost_unit_test_framework

This is what happens:

1. cmake -DCMAKE_BUILD_TYPE=Coverage .. build_coverage directory is created with a target.dir directory and *.gcno files in it
2. cd tests && make Test executable is created and linked against the shared library in the build_coverage directory (Maybe here is my mistake)
3. make coverage Coverage data is collected and tests are executed