Appveyor: manually add zstd flags to configure

            Set-Location "${env:build}"

            Execute-Bash "which ${env:target}-gcc"

            Execute-Bash "${env:target}-gcc --version"

            Execute-Bash "../configure --prefix=/${env:compiler_path} --build=${env:target} --host=${env:target} --disable-asciidoc --enable-fatal-warnings ${env:hardening}"

            Execute-Bash "ZSTD_CFLAGS='-L/${env:compiler_path}/include' ZSTD_LIBS='-L/${env:compiler_path}/lib -lzstd' ../configure --prefix=/${env:compiler_path} --build=${env:target} --host=${env:target} --disable-asciidoc --enable-fatal-warnings ${env:hardening}"

            Execute-Bash "V=1 make -j2"

            Execute-Bash "V=1 make -j2 install"

     }

  o Minor bugfixes (continuous integration, Windows):

    - Manually configure the zstd compiler options, when building using

      mingw on Appveyor Windows CI. The MSYS2 mingw zstd package does not

      come with a pkg-config file. Fixes bug 28454; bugfix on 0.3.4.1-alpha.