treat qwt headers as system headers to silence warnings

CMakeLists.txt

@@ -13,8 +13,11 @@ set(PROJECT_VERSION ${PROJECT_VERSION_STRING})
 add_compile_definitions(SLS_DET_VERSION="${PROJECT_VERSION}")
 
 set(CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_STANDARD 17)
+    set(CMAKE_CXX_STANDARD_REQUIRED ON)
+endif()
 
 cmake_policy(SET CMP0074 NEW)
 

libs/qwt/CMakeLists.txt

@@ -38,7 +38,7 @@ find_package(Qt5 REQUIRED
 file(GLOB QWT_SOURCES ${QWTBASE}src/*.cpp)
 file(GLOB QWT_HEADERS ${QWTBASE}src/*.h)
 add_library(qwt STATIC ${QWT_SOURCES} ${QWT_HEADERS} )
-target_include_directories(qwt PUBLIC 
+target_include_directories(qwt SYSTEM PUBLIC 
 	$<BUILD_INTERFACE:${QWTBASE}/src>
 )
 
@@ -54,4 +54,4 @@ target_link_libraries(qwt PUBLIC
 
 
 # Disable all warnings in QWT since we don’t want to patch the library
-target_compile_options(qwt PRIVATE -w)
+target_compile_options(qwt PRIVATE -w)