Resolve "Issue with hard-coded root build-dir"

.rootlogon.C

+#include <fmt/core.h>
+
+void _rootlogon() {
+  // ensure we have format loaded. Should be able to move to C++20 fmt library
+  // some point in the future
+  R__LOAD_LIBRARY(libfmt);
+
+  // check if we requested a special build directory through the environment,
+  const char*       build_dir_from_env = gSystem->Getenv("ROOT_BUILD_TMPDIR");
+  const std::string build_dir = build_dir_from_env ? build_dir_from_env : "$HOME/.root_build_dir";
+
+  // ensure the build directory exists to avoid crashes due a directory
+  // note: in principle this allows us to execute arbitrary code, but seeing
+  //       as this can only be manipulated through the shell environment in the
+  //       first place this should not be a safety concern.
+  const std::string mkdir = fmt::format("mkdir -p {}", build_dir);
+  gSystem->Exec(mkdir.c_str());
+
+  // Communicate what build directory we are using
+  fmt::print("---> Setting ACLiC build directory to: {}\n\n", build_dir);
+  gSystem->SetBuildDir(build_dir.c_str());
+
+  // Ensure we have our environment setup
+  gROOT->ProcessLine(".include /usr/local/include/hcana");
+  gROOT->ProcessLine(".include SCRIPTS/include");
+  gROOT->ProcessLine(".include .");
+
+  // Some basic plotting setup
+  gROOT->SetStyle("Plain");
+  gStyle->SetPalette(1);
+
+  // Load extra ROOT libraries, TODO check if this is still
+  // needed in the newer ROOT versions we are using.
+  gSystem->Load("libTree");
+  gSystem->Load("libTreePlayer");
+  gSystem->Load("libHist");
+}

rootlogon.C

-void rootlogon() {
-  gSystem->SetBuildDir("$HOME/.root_build_dir");
-  gROOT->ProcessLine(".include /usr/local/include/hcana");
-  gROOT->ProcessLine(".include SCRIPTS/include");
-  gROOT->ProcessLine(".include .");
-  R__LOAD_LIBRARY(libfmt)
-  gROOT->SetStyle("Plain");
-  gStyle->SetPalette(1);
-  gSystem->Load("libTree");
-  gSystem->Load("libTreePlayer");
-  gSystem->Load("libHist");
-}