drop need to upgrade qt, 5.15 is by default installed; add CXX_FLAGS; retarget SYSROOT to 10.15

CI/before_install.osx.sh

@@ -2,7 +2,6 @@
 
 brew update
 brew outdated pkgconfig || brew upgrade pkgconfig
-brew install qt
 brew install ccache
 
 curl -fSL -R -J https://downloads.openmw.org/osx/dependencies/openmw-deps-ef2462c.zip -o ~/openmw-deps.zip

CI/before_script.osx.sh

@@ -13,8 +13,9 @@ cmake \
 -D CMAKE_PREFIX_PATH="$DEPENDENCIES_ROOT;$QT_PATH" \
 -D CMAKE_C_COMPILER_LAUNCHER="$CCACHE_EXECUTABLE" \
 -D CMAKE_CXX_COMPILER_LAUNCHER="$CCACHE_EXECUTABLE" \
+-D CMAKE_CXX_FLAGS="-std=c++11 -stdlib=libc++" \
 -D CMAKE_OSX_DEPLOYMENT_TARGET="10.9" \
--D CMAKE_OSX_SYSROOT="macosx10.14" \
+-D CMAKE_OSX_SYSROOT="macosx10.15" \
 -D CMAKE_BUILD_TYPE=Release \
 -D OPENMW_OSX_DEPLOYMENT=TRUE \
 -D BUILD_ESMTOOL=FALSE \