Appveyor: manually add zstd flags to configure

The mingw zstd from MSYS2 doesn't come with a pkg-config file.

Fixes 28454 on Tor 0.3.4.1-alpha.

.appveyor.yml

@@ -43,7 +43,7 @@ build_script:
             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"
      }

changes/bug28454

@@ -0,0 +1,4 @@
+  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.