re-introduced Nils changed to cmake build interface that were lost in rebase

Author: DiamonDinoia

CMakeLists.txt

@@ -135,7 +135,8 @@ function(set_finufft_options target)
     endif ()
     target_compile_options(${target} PRIVATE $<$<CONFIG:Release>:${FINUFFT_CXX_FLAGS_RELEASE}>)
     target_compile_options(${target} PRIVATE $<$<CONFIG:RelWithDebInfo>:${FINUFFT_CXX_FLAGS_RELWITHDEBINFO}>)
-    target_include_directories(${target} PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/include")
+    target_include_directories(${target} PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
+    target_include_directories(${target} SYSTEM INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/include>)
     if (FINUFFT_USE_OPENMP)
         target_link_libraries(${target} PRIVATE OpenMP::OpenMP_CXX)
         # there are issues on windows with OpenMP and CMake, so we need to manually add the flags
@@ -202,7 +203,8 @@ if (FINUFFT_USE_CPU)
     if (NOT WIN32)
         target_link_libraries(finufft PUBLIC m)
     endif ()
-    target_include_directories(finufft PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/include")
+    target_include_directories(finufft PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
+    target_include_directories(finufft SYSTEM INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/include>)
 
     add_library(finufft_static STATIC src/utils_precindep.cpp contrib/legendre_rule_fast.cpp)
     set_finufft_options(finufft_static)
@@ -211,7 +213,8 @@ if (FINUFFT_USE_CPU)
     if (NOT WIN32)
         target_link_libraries(finufft_static PUBLIC m)
     endif ()
-    target_include_directories(finufft_static PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/include")
+    target_include_directories(finufft_static PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
+    target_include_directories(finufft_static SYSTEM INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_PREFIX}/include>)
 
     file(GLOB FINUFFT_PUBLIC_HEADERS "${CMAKE_CURRENT_SOURCE_DIR}/include/finufft*.h")
     set_target_properties(finufft PROPERTIES PUBLIC_HEADER "${FINUFFT_PUBLIC_HEADERS}")