OSS-Fuzz: Don't include GIF reader in cjpeg fuzzer

This version of libjpeg-turbo does not support compressing GIF input
files, and attempting to do so caused the cjpeg fuzzer to exit
unexpectedly (which OSS-Fuzz considers to be a crash.)

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=47087
diff --git a/fuzz/CMakeLists.txt b/fuzz/CMakeLists.txt
index 9f044c6..6083c87 100644
--- a/fuzz/CMakeLists.txt
+++ b/fuzz/CMakeLists.txt
@@ -26,7 +26,8 @@
 message(STATUS "C++ Compiler flags = ${EFFECTIVE_CXX_FLAGS}")
 
 add_executable(cjpeg_fuzzer${FUZZER_SUFFIX} cjpeg.cc ../cdjpeg.c ../rdbmp.c
-  ../rdgif.c ../rdppm.c ../rdswitch.c ../rdtarga.c)
+  ../rdppm.c ../rdswitch.c ../rdtarga.c)
+string(REGEX REPLACE "-DGIF_SUPPORTED " "" COMPILE_FLAGS ${COMPILE_FLAGS})
 set_property(TARGET cjpeg_fuzzer${FUZZER_SUFFIX} PROPERTY COMPILE_FLAGS
   ${COMPILE_FLAGS})
 target_link_libraries(cjpeg_fuzzer${FUZZER_SUFFIX} ${FUZZ_LIBRARY} jpeg-static)