Externals Update, master branch (2019.10.29.)

This MR updates all projects of the master branch to atlasexternals-2.0.44.

The updates compared to the currently used tag (atlasexternals@2.0.43...2.0.44) are as follows:

• Updated the code to build AthenaExternals successfully using Clang 8 set up from /cvmfs/sft.cern.ch (see ATLINFR-2904);
• Updated the Gdb build to turn off the usage of makeinfo if the texinfo package was not installed on the build machine (see ATLINFR-3233).

The updates in the configuration of some of the packages is to make it possible to fully configure a build against the x86_64-centos7-clang8-opt build of LCG_96. (For that platform LCG_96 doesn't provide the gperftools package for some reason...)

Unfortunately modern CMake versions just refuse to generate a build configuration if an external is not found. 😦 In the past CMake would still generate the Makefiles, it would just print a bunch of error messages while doing so. But now the build system generation never happens. So I had to add these checks in the code.

The update in IOVDbSvc was just because the find_package(CURL REQUIRED) call in there has hurt me a number of times already. And we should really not be using the REQUIRED keyword in the configuration of the packages... (I'll have to follow up with @desalvo on this, that curl should be added to the CentOS7 Singularity/Docker images. As currently it's not possible to build Athena inside those images because of this...)