Makefile: Prepend to LD_LIBRARY_PATH, instead of overwriting it

When compiling with some-gcc which needs /some-gcc-lib/libxyz.so.1, a way to
compile gcc-python-plugin is to write a wrapper some-gcc.sh:
...
export LD_LIBRARY_PATH=/some-gcc-lib:$LD_LIBRARY_PATH
some-gcc "$@"
...
and use "make CC=some-gcc.sh", which works fine.

However, when we try without wrapper:
...
export LD_LIBRARY_PATH=/some-gcc-lib:$LD_LIBRARY_PATH
make CC=some-gcc
...
we run into:
...
some-gcc/../../lib64/gcc/x86_64-linux/8/cc1: error while loading shared \
  libraries: libxyz.so.1: cannot open shared object file: No such file or \
  directory
...
because this overwrites LD_LIBRARY_PATH, rather than prepending:
...
LD_LIBRARY_PATH=gcc-c-api CC=some-gcc python run-test-suite.py
...

Fix INVOCATION_ENV_VARS in Makefile to prepend to LD_LIBRARY_PATH.

Author: vries

Makefile

@@ -132,7 +132,7 @@ plugin: autogenerated-config.h $(PLUGIN_DSO)
 # When running the plugin from a working copy, use LD_LIBARY_PATH=gcc-c-api
 # so that the plugin can find its libgcc-c-api.so there
 #
-INVOCATION_ENV_VARS := LD_LIBRARY_PATH=gcc-c-api CC=$(CC)
+INVOCATION_ENV_VARS := LD_LIBRARY_PATH=gcc-c-api:$(LD_LIBRARY_PATH) CC=$(CC)
 
 # When installing, both the plugin and libgcc-c-api.so will be installed to
 # $(GCCPLUGINS_DIR), so we give the plugin an RPATH of $(GCCPLUGINS_DIR)