gcc 7 fails on -Wunused-const-variable for Zstd headers

When using gcc 7 on macOS:

$ gcc-7 --version
gcc-7 (Homebrew GCC 7.3.0) 7.3.0

I see the following errors:

In file included from ../src/common/compress_zstd.c:29:0:
/usr/local/Cellar/zstd/1.3.4//include/zstd.h:593:29: error: 'ZSTD_defaultCMem' defined but not used [-Werror=unused-const-variable=]
 static ZSTD_customMem const ZSTD_defaultCMem = { NULL, NULL, NULL };  /**< this constant defers to stdlib's functions */
                             ^~~~~~~~~~~~~~~~
/usr/local/Cellar/zstd/1.3.4//include/zstd.h:404:21: error: 'ZSTD_skippableHeaderSize' defined but not used [-Werror=unused-const-variable=]
 static const size_t ZSTD_skippableHeaderSize = 8;  /* magic number + skippable frame length */
                     ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/zstd/1.3.4//include/zstd.h:403:21: error: 'ZSTD_frameHeaderSize_max' defined but not used [-Werror=unused-const-variable=]
 static const size_t ZSTD_frameHeaderSize_max = ZSTD_FRAMEHEADERSIZE_MAX;
                     ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/zstd/1.3.4//include/zstd.h:402:21: error: 'ZSTD_frameHeaderSize_min' defined but not used [-Werror=unused-const-variable=]
 static const size_t ZSTD_frameHeaderSize_min = ZSTD_FRAMEHEADERSIZE_MIN;
                     ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/zstd/1.3.4//include/zstd.h:401:21: error: 'ZSTD_frameHeaderSize_prefix' defined but not used [-Werror=unused-const-variable=]
 static const size_t ZSTD_frameHeaderSize_prefix = ZSTD_FRAMEHEADERSIZE_PREFIX;
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~

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