lgtm: set TMPDIR to /var/tmp

`/tmp` has `noexec` set in the LGTM infrastructure which breaks meson's
compilation checks. Let's use `/var/tmp` for now, until the issue is
properly resolved

See: https://discuss.lgtm.com/t/meson-fails-to-determine-sizes-of-basic-data-types/2639

.lgtm.yml

@@ -1,3 +1,6 @@
+---
+# vi: ts=2 sw=2 et:
+
 extraction:
   cpp:
     prepare:
@@ -12,6 +15,16 @@ extraction:
     after_prepare:
       - pip3 install meson
       - export PATH="$HOME/.local/bin/:$PATH"
+    before_index:
+      # /tmp has `noexec` set in the LGTM infra which breaks meson's
+      # compilation checks. Let's temporarily override TMPDIR to /var/tmp which
+      # should work as expected
+      # See: https://discuss.lgtm.com/t/meson-fails-to-determine-sizes-of-basic-data-types/2639
+      - export TMPDIR=/var/tmp
+    index:
+      build_command:
+        - meson build
+        - ninja -C build
   python:
     python_setup:
       version: 3