Configure CMake to require available backends on CI services

Configure CMake to require the installed backend libraries.  This should
help to find build system regressions.

On TravisCI with Linux this requires the ALSA, PulseAudio, PortAudio, OSS
and JACK backend.

On TravisCI with Android cross compile Linux this requires the OpenSL
backend.

On TravisCI with MacOSX this requires the CoreAudio backend.

ON AppVeyor with Windows this requires the WinMM, DSound and MMDevAPI
backend.

.travis.yml

@@ -31,16 +31,25 @@ install:
 script:
   - >
     if [[ "${TRAVIS_OS_NAME}" == "linux" && -z "${BUILD_ANDROID}" ]]; then
-      cmake .
+      cmake \
+        -DALSOFT_REQUIRE_ALSA=ON \
+        -DALSOFT_REQUIRE_OSS=ON \
+        -DALSOFT_REQUIRE_PORTAUDIO=ON \
+        -DALSOFT_REQUIRE_PULSEAUDIO=ON \
+        -DALSOFT_REQUIRE_JACK=ON \
+        .
     fi
   - >
     if [[ "${TRAVIS_OS_NAME}" == "linux" && "${BUILD_ANDROID}" == "true" ]]; then
       cmake \
         -DCMAKE_TOOLCHAIN_FILE=~/android-ndk-r14/build/cmake/android.toolchain.cmake \
+        -DALSOFT_REQUIRE_OPENSL=ON \
         .
     fi
   - >
     if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then
-      cmake .
+      cmake \
+        -DALSOFT_REQUIRE_COREAUDIO=ON \
+        .
     fi
   - make -j2

appveyor.yml

@@ -14,6 +14,6 @@ install:
 
 build_script:
     - cd build
-    - cmake .. -G"%GEN%"
+    - cmake -G"%GEN%" -DALSOFT_REQUIRE_WINMM=ON -DALSOFT_REQUIRE_DSOUND=ON -DALSOFT_REQUIRE_MMDEVAPI=ON ..
     - cmake --build . --config %CFG% --clean-first