Use gcc-8 on Ubuntu 18.04

To address #153 on Ubuntu 18.04.

builder/Dockerfile.ubuntu-1804

@@ -7,7 +7,8 @@ RUN set -x \
   && export DEBIAN_FRONTEND=noninteractive \
   && apt-get update \
   && apt-get install -y curl libcurl4-openssl-dev libicu-dev libopenblas-base libpcre2-dev wget python-pip \
-  && apt-get build-dep -y r-base
+  && apt-get build-dep -y r-base \
+  && apt-get install -y gcc-8 g++-8 gfortran-8
 
 RUN pip install awscli
 

builder/build.sh

@@ -120,6 +120,13 @@ compile_r() {
     --with-blas \
     --with-lapack"
 
+  if _version_is_greater_than ${R_VERSION} 4.2.10; then
+      if [[ "${OS_IDENTIFIER}" = "ubuntu-1804" ]]; then
+          default_configure_options="$default_configure_options \
+            CC=gcc-8 CXX=g++-8 FC=gfortran-8"
+      fi
+  fi
+
   CONFIGURE_OPTIONS=${CONFIGURE_OPTIONS:-$default_configure_options}
 
   # set some common environment variables for the configure step

builder/package.ubuntu-1804

@@ -27,9 +27,9 @@ deb:
   fields:
     Bugs: https://github.com/rstudio/r-builds/issues
 depends:
-- g++
+- g++-8
-- gcc
+- gcc-8
-- gfortran
+- gfortran-8
 - libbz2-dev
 - libc6
 - libcairo2