kiss: give build scripts some environment variables

- Defines DESTDIR so you no longer need to export it yourself.
- Defines CC and CXX so all builds will now work with other
  compilers if gcc/g++ are the default when unset.
- Defines KISS_ROOT with the sanitized version used by the package
  manager. It can now be safely joined with other paths.
  ("$KISS_ROOT/path/to/file")

kiss

@@ -804,13 +804,27 @@ pkg_build() {
 
     # Call the build script, log the output to the terminal and to a file.
     # There's no PIPEFAIL in POSIX shell so we must resort to tricks like kill.
-    { "$repo_dir/build" "$pkg_dir/$1" "$repo_ver" 2>&1 || {
-        log "$1" "Build failed"
-        log "$1" "Log stored to $log_dir/$1-$time-$pid"
-        run_hook build-fail "$pkg" "$pkg_dir/$1"
-        pkg_clean
-        kill 0
-    } } | tee "$log_dir/$1-$time-$pid"
+    {
+        set -a
+
+        # Give the script a modified environment. Define CC and CXX giving them
+        # the generic values cc and c++. Define KISS_ROOT sanitized to ensure
+        # safe path joining in build files.
+        CC="${CC:-cc}" \
+        CXX="${CXX:-c++}" \
+        DESTDIR="$pkg_dir/$1" \
+        KISS_ROOT="$KISS_ROOT" \
+
+        "$repo_dir/build" "$pkg_dir/$1" "$repo_ver" 2>&1 || {
+            log "$1" "Build failed"
+            log "$1" "Log stored to $log_dir/$1-$time-$pid"
+            run_hook build-fail "$pkg" "$pkg_dir/$1"
+            pkg_clean
+            kill 0
+        }
+
+        set +a
+    } | tee "$log_dir/$1-$time-$pid"
 
     # Delete the log file if the build succeeded to prevent the directory
     # from filling very quickly with useless logs.