A GCC-specific setting from years ago. gcc-9 quietly ignores it (the
resulting binaries appear to be the same whether -fno-ipa-sra has been
specified or not), clang fails on it as not recognised.
Just get rid of it altogether.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -100,7 +100,7 @@
 if(APPLE)
     list(APPEND GMIC_CXX_COMPILE_FLAGS -mmacosx-version-min=10.8 -stdlib=libc++ -Wno-error=c++11-narrowing -Wc++11-extensions -fpermissive)
 else()
-    list(APPEND GMIC_CXX_COMPILE_FLAGS -Wno-error=narrowing -fno-ipa-sra -fpermissive)
+    list(APPEND GMIC_CXX_COMPILE_FLAGS -Wno-error=narrowing -fpermissive)
 endif()
 
 if(NOT "${PRERELEASE_TAG}" STREQUAL "")