Install GCC when testing basemap on Python 2.7

Unfortunately, this is required because `matplotlib` relies internally
on the `subprocess32` backport, which needs to be compiled, and for
which no pre-compiled manylinux1 binary wheels are available.

Author: molinav

.github/workflows/basemap-for-manylinux.yml

@@ -246,6 +246,11 @@ jobs:
         with:
           name: artifacts-build-${{ matrix.arch }}-${{ matrix.python-version }}
           path: ${{ env.PKGDIR }}/dist
+      -
+        name: Install GCC toolchain
+        run: |
+          apt-get update
+          apt-get install -y gcc g++ make
       -
         name: Install package
         run: |