build/bin/sage-get-system-packages: Add/update comments

Author: Matthias Koeppe

build/bin/sage-get-system-packages

@@ -49,18 +49,24 @@ for PKG_BASE in $SPKGS; do
 
     case "$SYSTEM:$ENABLE_SYSTEM_SITE_PACKAGES" in
         install-requires*|pip*)
+            # This is output for installation of packages into a Python environment.
+            # So it's OK to use any Python packages.
             ;;
         *:)
-            # Skip this package if it uses the SAGE_PYTHON_PACKAGE_CHECK
-            # macro and if --enable-system-site-packages was NOT passed
-            # to ./configure (or if ./configure has not yet been run).
+            # --enable-system-site-packages was NOT passed to configure
+            # (or script was run outside of the sage-build-env).
+            #
+            # Skip this package if it uses the SAGE_PYTHON_PACKAGE_CHECK macro.
+            #
             SPKG_CONFIGURE="${SAGE_ROOT}/build/pkgs/${PKG_BASE}/spkg-configure.m4"
             if grep -q SAGE_PYTHON_PACKAGE_CHECK "${SPKG_CONFIGURE}" 2>/dev/null;
             then
                 continue
             fi
             ;;
         *)
+            # --enable-system-site-packages was passed to configure.
+            # So any package (Python, non-Python) is fine.
             ;;
     esac