Remove superfluous CMake flag in AppVeyor build

The source directory already defaults to the current directory.

.appveyor.yml

@@ -30,14 +30,14 @@ for:
             - GENERATOR: MinGW Makefiles
     build_script:
         - set PATH=%PATH:C:\Program Files\Git\usr\bin=C:\MinGW\bin%
-        - cmake -S . -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
+        - cmake -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
         - cmake --build build
 -
     matrix:
         only:
             - GENERATOR: Visual Studio 12 2013
     build_script:
-        - cmake -S . -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
+        - cmake -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
         - cmake --build build --target glfw
 notifications:
     - provider: Email