Pass SKBUILD to the SimpleITK external projects

Enable SimpleITK to choose which Python install to used based on
SKBUILD being defined.

Author: blowekamp

CMakeLists.txt

@@ -138,6 +138,7 @@ if(SimpleITKPythonPackage_SUPERBUILD)
         -DSimpleITK_BUILD_STRIP:BOOL=ON
         -DSimpleITK_BUILD_DISTRIBUTE:BOOL=ON
         -DSITK_GIT_PROTOCOL:STRING=git
+        -DSKBUILD:BOOL=${SKBUILD}
       USES_TERMINAL_CONFIGURE 1
       USES_TERMINAL_BUILD 1
       INSTALL_COMMAND ""
@@ -220,6 +221,7 @@ if(SimpleITKPythonPackage_SUPERBUILD)
       -DSimpleITK_PYTHON_THREADS:BOOL=${SimpleITK_PYTHON_THREADS}
       -DSimpleITK_DIR:PATH=${SimpleITK_DIR}
       -DSWIG_EXECUTABLE:PATH=${SWIG_EXECUTABLE}
+      -DSKBUILD:BOOL=ON
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     INSTALL_COMMAND ""