ARROW-5697: [GLib] Use system pkg-config in c_glib/Dockerfile to corr…

…ectly find system libraries such as libglib

The conda-provided pkg-config was being used

Author: Wes McKinney <wesm+git@apache.org>
Author: Sutou Kouhei <kou@clear-code.com>

Closes #4664 from wesm/docker-glib-build and squashes the following commits:

ae66d22 <Sutou Kouhei> Ensure using system pkg-config
d88c9f6 <Wes McKinney> Try to fix c_glib Dockerfile

c_glib/Dockerfile

@@ -30,7 +30,7 @@ RUN apt-get update -y -q && \
     && rm -rf /var/lib/apt/lists/*
 
 COPY c_glib/Gemfile /arrow/c_glib/
-RUN conda install meson=0.47.1 && \
+RUN conda install meson=0.50.1 && \
     conda clean --all && \
     gem install bundler && \
     bundle install --gemfile arrow/c_glib/Gemfile
@@ -40,6 +40,7 @@ ENV ARROW_BUILD_TESTS=OFF \
     ARROW_BUILD_UTILITIES=OFF \
     ARROW_INSTALL_NAME_RPATH=OFF \
     LD_LIBRARY_PATH="${CONDA_PREFIX}/lib" \
+    PKG_CONFIG=/usr/bin/pkg-config \
     PKG_CONFIG_PATH="${CONDA_PREFIX}/lib/pkgconfig" \
     GI_TYPELIB_PATH="${CONDA_PREFIX}/lib/girepository-1.0"