Use existing CMake library in a new project including its headers - c++

Question

I'm working on a project where I have an existing CMake library (https://github.com/OpenEtherCATsociety/SOEM) stored in project/lib/SOEM/.
The source for my application lives in project/src/
I have a root CMakeLists.txt which adds both subdirectories.
Now my question is how do I link the SOEM library to my executable and also use its headers without relative paths?
SOEM contains its own CMakeLists.txt which I would preferably not touch at all if that's possible. It builds a library and installs it together with its headers, however, I don't know how to make use of the latter.
cmake_minimum_required(VERSION 2.8.4)
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules")
project(SOEM C)
if (CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
# Default to installing in SOEM source directory
set(CMAKE_INSTALL_PREFIX ${CMAKE_SOURCE_DIR}/install)
endif()
set(SOEM_INCLUDE_INSTALL_DIR include/soem)
if(WIN32)
set(OS "win32")
include_directories(oshw/win32/wpcap/Include)
link_directories(${CMAKE_SOURCE_DIR}/oshw/win32/wpcap/Lib)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /D _CRT_SECURE_NO_WARNINGS")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /WX")
set(OS_LIBS wpcap.lib Packet.lib Ws2_32.lib Winmm.lib)
elseif(UNIX)
set(OS "linux")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Werror")
set(OS_LIBS pthread rt)
elseif(${CMAKE_SYSTEM_NAME} MATCHES "rt-kernel")
set(OS "rtk")
message("ARCH is ${ARCH}")
message("BSP is ${BSP}")
include_directories(oshw/${OS}/${ARCH})
file(GLOB OSHW_EXTRA_SOURCES oshw/${OS}/${ARCH}/*.c)
set(OSHW_SOURCES "${OS_HW_SOURCES} ${OSHW_ARCHSOURCES}")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Werror")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-but-set-variable")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-function")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-format")
set(OS_LIBS "-Wl,--start-group -l${BSP} -l${ARCH} -lkern -ldev -lsio -lblock -lfs -lusb -llwip -leth -li2c -lrtc -lcan -lnand -lspi -lnor -lpwm -ladc -ltrace -lc -lm -Wl,--end-group")
endif()
message("OS is ${OS}")
file(GLOB SOEM_SOURCES soem/*.c)
file(GLOB OSAL_SOURCES osal/${OS}/*.c)
file(GLOB OSHW_SOURCES oshw/${OS}/*.c)
file(GLOB SOEM_HEADERS soem/*.h)
file(GLOB OSAL_HEADERS osal/osal.h osal/${OS}/*.h)
file(GLOB OSHW_HEADERS oshw/${OS}/*.h)
include_directories(soem)
include_directories(osal)
include_directories(osal/${OS})
include_directories(oshw/${OS})
add_library(soem STATIC
${SOEM_SOURCES}
${OSAL_SOURCES}
${OSHW_SOURCES}
${OSHW_EXTRA_SOURCES})
target_link_libraries(soem ${OS_LIBS})
install(TARGETS soem DESTINATION lib)
install(FILES
${SOEM_HEADERS}
${OSAL_HEADERS}
${OSHW_HEADERS}
DESTINATION ${SOEM_INCLUDE_INSTALL_DIR})
add_subdirectory(test/linux/slaveinfo)
add_subdirectory(test/linux/eepromtool)
add_subdirectory(test/linux/simple_test)