scripts: Set OSX_SYSROOT for 10.13

So our build are compatible with TravisCI, which currently defaults to
using macOS 10.13.

Author: thewtex

scripts/macpython-build-wheels.sh

@@ -99,6 +99,7 @@ for VENV in "${VENVS[@]}"; do
         -DITK_BINARY_DIR:PATH=${build_path} \
         -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=${osx_target} \
         -DCMAKE_OSX_ARCHITECTURES:STRING=x86_64 \
+        -DCMAKE_OSX_SYSROOT:STRING=macosx10.13 \
         -DITK_WRAP_unsigned_short:BOOL=ON \
         -DPYTHON_EXECUTABLE:FILEPATH=${PYTHON_EXECUTABLE} \
         -DPYTHON_INCLUDE_DIR:PATH=${PYTHON_INCLUDE_DIR} \
@@ -124,6 +125,7 @@ for VENV in "${VENVS[@]}"; do
           -DBUILD_TESTING:BOOL=OFF \
           -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=${osx_target} \
           -DCMAKE_OSX_ARCHITECTURES:STRING=x86_64 \
+          -DCMAKE_OSX_SYSROOT:STRING=macosx10.13 \
           -DITK_WRAP_unsigned_short:BOOL=ON \
           -DPYTHON_EXECUTABLE:FILEPATH=${PYTHON_EXECUTABLE} \
           -DPYTHON_INCLUDE_DIR:PATH=${PYTHON_INCLUDE_DIR} \
@@ -151,6 +153,7 @@ for VENV in "${VENVS[@]}"; do
           -DITK_BINARY_DIR:PATH=${build_path} \
           -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=${osx_target} \
           -DCMAKE_OSX_ARCHITECTURES:STRING=x86_64 \
+          -DCMAKE_OSX_SYSROOT:STRING=macosx10.13 \
           -DITKPythonPackage_ITK_BINARY_REUSE:BOOL=ON \
           -DITKPythonPackage_WHEEL_NAME:STRING=${wheel_name} \
           -DITK_WRAP_unsigned_short:BOOL=ON \