Setting the make -j (jobs) flag based on the

`ASDF_CONCURRENCY` environment variable

See:
 - asdf-vm/asdf#57
 - asdf-vm/asdf#75
 - asdf-vm/asdf#147

bin/install

@@ -15,6 +15,7 @@ install_php() {
 
     local source_path=$(get_download_file_path $install_type $version $tmp_download_dir)
     local configure_options="$(construct_configure_options $install_path)"
+    local make_flags="-j$ASDF_CONCURRENCY"
 
     download_source $install_type $version $source_path
 
@@ -32,9 +33,9 @@ install_php() {
         # Build PHP
         ./buildconf --force || exit 1
         ./configure $configure_options || exit 1
-        make || exit 1
-        # make test || exit 1
-        make install || exit 1
+        make "$make_flags" || exit 1
+        # make "$make_flags" test || exit 1
+        make "$make_flags" install || exit 1
     )
 }