older gccs don't accept -Wunused-const-variable

In src/common/compress_zstd.c, we use the DISABLE_GCC_WARNING() macro to disable the -Wunused-const-variable warning. Some versions of gcc don't recognize that warning option, so they will warn about incorrect pragma usage.

src/common/compress_zstd.c:29:1: warning: unknown option after ‘#pragma GCC diagnostic’ kind [-Wpragmas]
 DISABLE_GCC_WARNING(unused-const-variable)
 ^

We should probably define a macro if -Wunused-const-variable is supported by gcc and update the preprocessor conditional in compress_zstd.c

#ifdef HAVE_ZSTD
DISABLE_GCC_WARNING(unused-const-variable)

to use it.

Reported on IRC.

Confirmed on Xenial with gcc 5.4.0-6ubuntu1~~16.04.10 and libzstd1-dev 1.3.1+dfsg-1~~ubuntu0.16.04.1. Travis is still on Trusty so doesn't have a libzstd-dev.

This might be a regression on platforms with libzstd but an older gcc.

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