Fix download_if_necessary.sh oversight of tar ball

This commit fixes a regression that precluded recognizing a previously
downloaded tar ball so that packages were being downloaded even if the
corresponding source tar ball already existed in the default download path.

Author: Damian Rouson

doc/dependency_tree/opencoarrays/mpich-3.1.4/gcc-6.1.0/flex-2.6.0/bison-3.0.4/m4-1.4.17/.gitkeep

@@ -0,0 +1 @@
+

doc/dependency_tree/opencoarrays/mpich-3.1.4/gcc-6.1.0/gmp/.gitkeep

@@ -0,0 +1 @@
+

doc/dependency_tree/opencoarrays/mpich-3.1.4/gcc-6.1.0/mpc/.gitkeep

@@ -0,0 +1 @@
+

doc/dependency_tree/opencoarrays/mpich-3.1.4/gcc-6.1.0/mpfr/.gitkeep

@@ -0,0 +1 @@
+

prerequisites/build-functions/download_if_necessary.sh

@@ -1,16 +1,16 @@
 # shellcheck source=./ftp-url.sh
-source "${OPENCOARRAYS_SRC_DIR:-}/prerequisites/build-functions/ftp-url.sh"
+source "${opencoarrays_src_dir}/prerequisites/build-functions/ftp-url.sh"
 # shellcheck source=./set_SUDO_if_needed_to_write_to_directory.sh
-source "${OPENCOARRAYS_SRC_DIR:-}/prerequisites/build-functions/set_SUDO_if_needed_to_write_to_directory.sh"
+source "${opencoarrays_src_dir}/prerequisites/build-functions/set_SUDO_if_needed_to_write_to_directory.sh"
 
 # Download pkg-config if the tar ball is not already in the present working directory
 # shellcheck disable=SC2154
 download_if_necessary()
 {
-  download_path="${PWD}/downloads"
+  download_path="${opencoarrays_src_dir}/prerequisites/downloads"
   set_SUDO_if_needed_to_write_to_directory "${download_path}"
-  if [ -f "$url_tail" ] || [ -d "$url_tail" ]; then
-    info "Found '${url_tail}' in ${PWD}."
+  if  [[ -f "${download_path}/${url_tail}" || -d "${download_path}/${url_tail}" ]] ; then
+    info "Found '${url_tail}' in ${download_path}."
     info "If it resulted from an incomplete download, building ${package_name} could fail."
     info "Would you like to proceed anyway? (Y/n)"
     read -r proceed