Fix the bug of loading shared libraries. (#561)

* Prepend the package directory path to the "LD_LIBRARY_PATH" environment variable.

* Fix the bug of getting "LD_LIBRARY_PATH" environment variable.

* Fix loading shared libraries on Linux.

Co-authored-by: Czarek Tomczak <cztomczak@users.noreply.github.com>

tools/installer/cefpython3.__init__.py

@@ -26,9 +26,13 @@
 
 package_dir = os.path.dirname(os.path.abspath(__file__))
 
-# This loads the libcef.so library for the subprocess executable.
-# On Mac it works without setting library paths.
-os.environ["LD_LIBRARY_PATH"] = package_dir
+# This loads the libcef.so library for the subprocess executable on Linux.
+# TODO: Use -Wl,-rpath=\$$ORIGIN in Makefile.
+ld_library_path = os.environ.get("LD_LIBRARY_PATH")
+if ld_library_path and ld_library_path.strip():
+    os.environ["LD_LIBRARY_PATH"] = package_dir + os.pathsep + ld_library_path
+else:
+    os.environ["LD_LIBRARY_PATH"] = package_dir
 
 # This env variable will be returned by cefpython.GetModuleDirectory().
 os.environ["CEFPYTHON3_PATH"] = package_dir