Later versions of libjpeg-turbo build with CMake; to build with
user-specified cflags, the user must supply these to CMake as
-DCMAKE_C_FLAGS (Spack's typical approach of injecting these flags
into the compiler wrapper invocation is insufficient in this case).
Currently libjpeg-turbo cannot be implemented as a CMakePackage
(and thereby take advantage of the flag_handler implementation it
provides) because not all versions of libjpeg-turbo use CMake, so
this adds a custom implementation of flag_handler and
flags_to_build_system_args to libjpeg-turbo.
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