Try using make -k in the Appveyor configuration

Frequently, when a patch fails, it has failures in several files.
Using the "-k" flag will let us learn all the compilation errors,
not just the first one that the compiler hits.

Based on a patch by rl1987.

Closes ticket 31372.

.appveyor.yml

@@ -83,8 +83,8 @@ build_script:
              # configure its flags. liblzma just works.
              #>
             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=${env:target} --with-openssl-dir=/${env:compiler_path} --disable-asciidoc --enable-fatal-warnings ${env:hardening}"
-            Execute-Bash "V=1 make -j2"
-            Execute-Bash "V=1 make -j2 install"
+            Execute-Bash "V=1 make -k -j2"
+            Execute-Bash "V=1 make -k -j2 install"
      }
 
 test_script:
@@ -95,7 +95,7 @@ test_script:
             $buildpath = @("C:\msys64\${env:compiler_path}\bin") + $oldpath
             $env:Path = $buildpath -join ';'
             Set-Location "${env:build}"
-            Execute-Bash "VERBOSE=1 make -j2 check"
+            Execute-Bash "VERBOSE=1 make -k -j2 check"
     }
 
 on_finish:

changes/ticket31372_appveyor

@@ -0,0 +1,4 @@
+  o Minor features (continuous integration):
+    - When building on Appveyor, pass the "-k" flag to make, so that
+      we are informed of all compilation failures, not just the first
+      one or two. Closes part of ticket 31372.