Add logic to install Xcode-CLT if needed

 On macOS, add logic to install script that will install Xcode command
 line tools. Most logic cribbed from Homebrew installer.

 Fixes #325

Author: zbeekman

install.sh

@@ -234,6 +234,9 @@ source $opencoarrays_src_dir/prerequisites/install-functions/build_opencoarrays.
 # shellcheck source=./prerequisites/install-functions/report_results.sh
 source $opencoarrays_src_dir/prerequisites/install-functions/report_results.sh
 
+# shellcheck source=./prerequisites/install-functions/install-xcode-clt.sh
+source "${opencoarrays_src_dir}/prerequisites/install-functions/install-xcode-clt.sh"
+
 # ___________________ End of function definitions for use in the Main Body __________________
 
 
@@ -303,6 +306,8 @@ elif [[ "${arg_p:-}" == "opencoarrays" ]]; then
 
   else
 
+    # Install Xcode command line tools (CLT) if on macOS and if needed
+    maybe_install_xcodeCLT
     # Install OpenCoarrays
     cd prerequisites || exit 1
     installation_record=install-opencoarrays.log
@@ -320,12 +325,18 @@ elif [[ "${arg_p:-}" == "opencoarrays" ]]; then
 
 elif [[ "${arg_p:-}" == "ofp" ]]; then
 
+  # Install Xcode command line tools (CLT) if on macOS and if needed
+  maybe_install_xcodeCLT
+
   info "Invoking Open Fortran Parser build script with the following command:"
   info "\"${opencoarrays_src_dir}\"/prerequisites/install-ofp.sh"
   "${opencoarrays_src_dir}"/prerequisites/install-ofp.sh
 
 elif [[ ! -z "${arg_p:-}" ]]; then
 
+  # Install Xcode command line tools (CLT) if on macOS and if needed
+  maybe_install_xcodeCLT
+
   info "Invoking build script with the following command:"
   info "\"${opencoarrays_src_dir}\"/prerequisites/build.sh ${*:-}"
   "${opencoarrays_src_dir}"/prerequisites/build.sh "${@:-}"

prerequisites/install-functions/install-xcode-clt.sh

@@ -0,0 +1,70 @@
+# shellcheck shell=bash disable=SC2154,SC2148
+need_xcodeCLT() {
+  if [[ "${OSTYPE}" != [Dd][Aa][Rr][Ww][Ii][Nn]* ]]; then
+    echo "false"
+  else
+    local xcode_dir
+    xcode_dir="$(/usr/bin/xcode-select -print-path 2>/dev/null)"
+    if [[ ! -d "${xcode_dir}" || ! -x "${xcode_dir}/usr/bin/make" ]]; then
+      echo "true"
+    else
+      echo "false"
+    fi
+  fi
+}
+
+xcode_clt_install () {
+  info "It appears that you are on Mac OS and do not have the Xcode command line tools (CLT) installed."
+  app_store_label="$(softwareupdate -l | grep -B 1 -E "Command Line (Developer|Tools)" | awk -F"*" '/^ +\\*/ {print $2}' | sed 's/^ *//' | tail -n1)"
+  if [[ "${arg_y}" == "${__flag_present}" ]]; then
+    info "-y or --yes-to-all flag present. Proceeding with non-interactive download and install."
+  else
+    printf "Ready to install %s? (Y/n)" "${app_store_label}"
+    read -r install_xcodeclt
+    printf '%s\n' "${install_xcodeclt}"
+    if [[ "${install_xcodeclt}" == [nN]* ]]; then
+      emergency "${this_script}: Aborting: cannot procede without XCode CLT."
+    fi
+  fi
+  info "We will attempt to download and install XCode CLT for you now. Note: sudo priveledges required"
+  place_holder="/tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress"
+  sudo /usr/bin/touch "${place_holder}" || true
+  sudo /usr/sbin/softwareupdate -i app_store_label || true
+  sudo /bin/rm -f "${place_holder}" || true
+  sudo /usr/bin/xcode-select --switch "/Library/Developer/CommandLineTools" || true
+}
+
+headless_xcode_clt_install () {
+  # Fallback her if headless install failed
+  if [[ "${arg_y}" == "${__flag_present}" ]]; then
+    info "-y or --yes-to-all flag present. Proceeding with non-interactive download and install."
+  else
+    printf "Ready to install Xcode-CLT using xcode-select? (Y/n)"
+    read -r install_xcodeclt
+    printf '%s\n' "${install_xcodeclt}"
+    if [[ "${install_xcodeclt}" == [nN]* ]]; then
+      emergency "${this_script}: Aborting: cannot procede without XCode CLT."
+    fi
+  fi
+  info "Installing Xcode Command Line Tools (CLE), please follow instructions on popup window."
+  sudo /usr/bin/xcode-select --install || emergency "${this_script}: unable to run \`sudo xcode-select --install\`"
+  printf "Press <enter> once installation has completed"
+  read -r
+  sudo /usr/bin/xcode-select --switch "/Library/Developer/CommandLineTools" || \
+    emergency "${this_script}: Xcode-CLT installation and activation failed, unable to continue"
+}
+
+maybe_install_xcodeCLT () {
+  if [[ "$(need_xcodeCLT)" == "true" ]]; then
+    xcode_clt_install
+  fi
+  if [[ "$(need_xcodeCLT)" == "true" ]]; then
+    info "First Xcode-CLT installation failed, trying another method"
+    headless_xcode_clt_install
+  fi
+
+  clang_output="$(/usr/bin/xzrun clang 2>&1)" || true
+  if [[ "${clang_output}" =~ license ]]; then
+    emergency "${this_script}: It appears you have not agreed to the Xcode license. Please do so before attempting to run this script again. This may be acheived by opening Xcode.app or running \`sudo xcodebuild -license\`"
+  fi
+}