Set rpath on cxx-generate-packed-data so we don't need to set LD_LIBRARY_PATH

This commit sets the 'build_rpath' argument on the
cxx-generate-packed-data executable to the protobuf library's libdir,
which supports the use case where the protobuf library is installed into
a non-default path. For instance, in the CI environment protobuf is
installed into a subdirectory of $HOME, which is not searched by the
runtime linker.

This approach was recommended on the mesonbuild mailing list.

.travis.yml

@@ -19,7 +19,6 @@ env:
   global:
     - PROTOBUF_VERSION=3.6.1
     - PKG_CONFIG_PATH=$HOME/protobuf-$PROTOBUF_VERSION-bin/lib/pkgconfig
-    - LD_LIBRARY_PATH=$HOME/protobuf-$PROTOBUF_VERSION-bin/lib
 
 install:
     - pip3 install meson

meson.build

@@ -199,7 +199,8 @@ if get_option('build-compiler') and get_option('run-tests')
       executable('cxx-generate-packed-data', 't/generated-code2/cxx-generate-packed-data.cc',
         gen_protobuf.process('t/test-full.proto',
           preserve_path_from : meson.current_source_dir()),
-        dependencies : protobuf),
+        dependencies : protobuf,
+        build_rpath : protobuf.get_pkgconfig_variable('libdir')),
     capture : true)
 
   test('generated-code2',