gcc flags not appropriate

[Tux]

zstd/Makefile adds a truckload of CFLAGS that are only applicable if the compiler is GNU gcc, but cause epic failure when it is not, like when using HP C-ANSI-C on HP-UX.
Please guard that against non-gcc compilers. Our live is hard enough as it is and use $Config{cflags} instead. On HP-UX I would use something like -Ae +DD64 -z +Z (where +Z is the equiv of -fPIC)
FWIW this is in both Sereal::Encoder and Sereal::Decoder