How To Include External Libraries in CMake Project - c++

Question

I am confused on how to statically include the source code of SDL2. I am trying to do this to make a library I am working on more portable.
When I was setting this up as executable the library was compiled with it fine, but when I changed it to a library it wouldn't include the library.
Currently, when I try to include my library in another project it says "Cannot open include file: 'SDL2/SDL.h': No such file or directory". So it leads me to think that the cause of the error is that the include directories aren't exported with the static library.
My Filesystem:
include
--Header Files
src
--Source Files
extern
--SDL2
build
Here is an example of the file causing the error:
#include <iostream>
#include <SDL.h> //Error
using namespace std;
/* The code */
Here is an example of my main CMakeLists.txt:
cmake_minimum_required(VERSION 3.7)
set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
project(MyProject VERSION 1.0.0)
set(SDL2_SOURCE_DIR “${CMAKE_SOURCE_DIR}/extern/SDL2”)
add_subdirectory(extern/SDL2)
add_subdirectory(src)
Here is an example of my src CMakeLists.txt:
set(PROJECT_NAME MyProject)
file(GLOB HEADER_FILES "${CMAKE_SOURCE_DIR}/include/*.h")
file(GLOB SOURCES "*.cpp")
add_library(${PROJECT_NAME} ${SOURCES} ${HEADER_FILES})
target_include_directories(${PROJECT_NAME} PUBLIC "${CMAKE_SOURCE_DIR}/include" PUBLIC "${CMAKE_SOURCE_DIR}/extern/SDL2/include")
target_link_libraries(${PROJECT_NAME} PRIVATE SDL2main SDL2-static)
set_target_properties( ${PROJECT_NAME}
PROPERTIES
ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_SOURCE_DIR}/lib"
LIBRARY_OUTPUT_DIRECTORY "${CMAKE_SOURCE_DIR}/lib"
RUNTIME_OUTPUT_DIRECTORY "${CMAKE_SOURCE_DIR}/bin"
)

Answer 1

From sdl2 CMakeLists.txt try:
set(SDL_STATIC 1)
add_subdirectory(extern/SDL2) # And I recommend EXCLUDE_FROM_ALL
Also to be sure add a check:
foreach(i IN ITEMS SDL2main SDL2-static)
if(NOT TARGET ${i})
message(FATAL_ERROR "${i} is not a target")
endif()
endif()
target_link_libraries(${PROJECT_NAME} PRIVATE SDL2main SDL2-static)