Fix build with older openEXR versions

Dynamic exception specification is not allowed in c++-17. The enforcement needs
to be relaxed for the kimg_exr plugin when using openEXR versions older than 2.3.0.

(cherry picked from commit 3266a9c466)

src/imageformats/CMakeLists.txt

@@ -61,6 +61,11 @@ if(OpenEXR_FOUND)
     if(TARGET OpenEXR::OpenEXR)
         target_link_libraries(kimg_exr OpenEXR::OpenEXR)
     else()
+        if(OpenEXR_VERSION_STRING VERSION_LESS 2.3.0)
+            # Older OpenEXR versions use dynamic exception specifications, so
+            # cannot use C++17 with them
+            set_target_properties(kimg_exr PROPERTIES CXX_STANDARD 14)
+        endif()
         target_link_libraries(kimg_exr OpenEXR::IlmImf)
     endif()
     kde_target_enable_exceptions(kimg_exr PRIVATE)