Externals Update, master branch (2020.05.01.)

Switched all projects to atlasexternals-2.0.65.

The changes wrt. atlasexternals-2.0.64 are (atlasexternals@2.0.64...2.0.65):

• Forced GeoModelCore to be built with optimisations, even in Debug builds (thanks to @ssnyder);
• Updated the AtlasLCG modules to consistently set up <PACKAGE>_<executable>_EXECUTABLE variables for externals that provide executables (in preparation for atlasexternals!664 (closed));
• Updated FindXercesC.cmake to wrap CMake's own FindXercesC.cmake module (in preparation for atlasexternals!664 (closed));
• Updated GPerfTools to use Findgperftools.cmake instead of Findtcmalloc.cmake (in preparation for atlasexternals!664 (closed));
• Updated atlas_build_run.sh to handle quoted commands correctly (in preparation for atlasexternals!664 (closed));
• Fixed the build of Python 2.7 on systems that don't provide Python 2 in /usr (to fix the build of AnalysisBaseExternal on Ubuntu 20.04 );
• Updated GDB to version 9.1 (see ATLINFR-3516 and ATLINFR-3518);
• Command-line usage improvements in flake8_atlas (thanks to @fwinkl);
• Removed the EXTRA_PATTERNS argument from atlas_add_test(...) (thanks to @fwinkl).

Note that I believe this update should succeed in an incremental build as well. But I guess we'll see...