dev-lang/swift: Correctly merge extra build flags

The build script complains when the same option occurs multiple
times in the same preset, but merges them when they occur in separate
presets. Therefore restore the code that creates a stack of presets
with one for each subsequent argument, instead of packing them all
into a single preset.

Signed-off-by: Katalin Rebhan <me@dblsaiko.net>

dev-lang/swift/swift-6.0.3-r2.ebuild

@@ -214,16 +214,22 @@ src_configure() {
 
 	extra_build_flags+=(${SWIFT_EXTRA_BUILD_FLAGS})
 
-	if [[ ${#extra_build_flags[@]} -gt 0 ]]; then
-		SWIFT_BUILD_PRESET='gentoo,custom'
-		{
-			echo "[preset: gentoo,custom]"
-			echo "mixin-preset=gentoo"
-			for flag in "${extra_build_flags[@]}"; do
-				echo "${flag#--}"
-			done
-		} >> "${SWIFT_BUILD_PRESETS_INI_PATH}"
-	fi
+	local orig_preset="${SWIFT_BUILD_PRESET}"
+	local preset="${orig_preset}"
+	local n=1
+
+	{
+		for arg in "${extra_build_flags[@]}"; do
+			local next="${orig_preset},${n}"
+			printf '[preset: %s]\n' "${next}"
+			printf 'mixin-preset=%s\n' "${preset}"
+			echo "${arg#--}"
+			preset="${next}"
+			n="$((n + 1))"
+		done
+	} >> "${SWIFT_BUILD_PRESETS_INI_PATH}"
+
+	SWIFT_BUILD_PRESET="${preset}"
 }
 
 src_compile() {