Let appveyor test static and shared libs.

.appveyor_msys_build.sh

@@ -1,6 +1,6 @@
 export PATH=/c/msys64/mingw$ABI/bin:/c/projects/mpir/bin/:$PATH
 cd /c/projects/mpir
 ./autogen.sh
-./configure ABI=$ABI --with-system-yasm
+./configure ABI=$ABI $LIB
 make
 make check

appveyor.yml

@@ -6,8 +6,16 @@ environment:
   matrix:
     - COMPILER: MinGW-w64
       ABI: 32
+      LIB: --enable-static --disable-shared
+    - COMPILER: MinGW-w64
+      ABI: 32
+      LIB: --disable-static --enable-shared
     - COMPILER: MinGW-w64
       ABI: 64
+      LIB: --enable-static --disable-shared
+    - COMPILER: MinGW-w64
+      ABI: 64
+      LIB: --disable-static --enable-shared
     - BUILD_TYPE: Release
       COMPILER: MSVC15
       PLATFORM: x64