lgtm: fix the extraction process

As this project uses only Makefile, without any configuration step, and due to
a "non-standard" location of the source files, LGTM kept failing to find the
respective Makefile and build the sources. By tricking LGTM's build system
auto detection, that we use automake/configure, it correctly sets the source
dir, thus the compilation, extraction & analysis steps now work in the src/
subdirectory, as expected.

.lgtm.yml

@@ -0,0 +1,14 @@
+# vi: set ts=2 sw=2:
+extraction:
+  cpp:
+    prepare:
+      packages:
+        - libelf-dev
+        - pkg-config
+    after_prepare:
+      # As the buildsystem detection by LGTM is performed _only_ during the
+      # 'configure' phase, we need to trick LGTM we use a supported build
+      # system (configure, meson, cmake, etc.). This way LGTM correctly detects
+      # that our sources are in the src/ subfolder.
+      - touch src/configure
+      - chmod +x src/configure