appveyor: configure for Release with Debug Info

We build the library as debug with AppVeyor and package it this way,
which is unfortunate, because on Windows it's linked against debug
variants of the C runtime. Fix this by build RelWithDebInfo instead

Fixes #1128.

appveyor.yml

@@ -39,7 +39,7 @@ before_build:
   - cd build
 
 build_script:
-  - cmake -G %GENERATOR% -DSTATIC=ON -DSHARED=ON -DBUILD_EXAMPLES=%examples% -DBUILD_GAMES=%examples% -DINCLUDE_EVERYTHING=ON ..
+  - cmake -G %GENERATOR% -DCMAKE_BUILD_TYPE=RelWithDebInfo -DSTATIC=ON -DSHARED=ON -DBUILD_EXAMPLES=%examples% -DBUILD_GAMES=%examples% -DINCLUDE_EVERYTHING=ON ..
   - cmake --build . --target install
 
 after_build: