[PR PATCH] xbps-src: fix detecting subpackage availability

[paper42 <paper42@users.noreply.github.com>]

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There is a new pull request by paper42 against master on the void-packages repository

https://github.com/paper42/void-packages xbps-src-fix-subpkg-avail
https://github.com/void-linux/void-packages/pull/37617

xbps-src: fix detecting subpackage availability
<!-- Uncomment relevant sections and delete options which are not applicable -->

#### Testing the changes
- I tested the changes in this PR: **briefly**

This is marked as draft because it doesn't work as expected in all cases, for example when a subpackage is added like in cryptsetup:
```sh
if [ "$XBPS_TARGET_LIBC" = musl ]; then
	configure_args+=" --enable-static-cryptsetup"
	subpackages+=" cryptsetup-static"
...
```

```sh
%: ./xbps-src show-avail cryptsetup-static; echo $?
0
```

fixes #37578

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A patch file from https://github.com/void-linux/void-packages/pull/37617.patch is attached

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From f7f8cf709198a5c9b8d3287bc427b3969ca1cdae Mon Sep 17 00:00:00 2001
From: Michal Vasilek <michal@vasilek.cz>
Date: Mon, 20 Jun 2022 01:27:05 +0200
Subject: [PATCH] xbps-src: fix detecting subpackage availability

---
 common/xbps-src/shutils/pkgtarget.sh | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/common/xbps-src/shutils/pkgtarget.sh b/common/xbps-src/shutils/pkgtarget.sh
index ba8c63bca8aa..df7e4828b623 100644
--- a/common/xbps-src/shutils/pkgtarget.sh
+++ b/common/xbps-src/shutils/pkgtarget.sh
@@ -16,14 +16,21 @@ check_existing_pkg() {
 check_pkg_arch() {
     local cross="$1" _arch f match nonegation
 
-    if [ -n "$archs" ]; then
-        if [ -n "$cross" ]; then
-            _arch="$XBPS_TARGET_MACHINE"
-        elif [ -n "$XBPS_ARCH" ]; then
-            _arch="$XBPS_ARCH"
-        else
-            _arch="$XBPS_MACHINE"
+    if [ -n "$cross" ]; then
+        _arch="$XBPS_TARGET_MACHINE"
+    elif [ -n "$XBPS_ARCH" ]; then
+        _arch="$XBPS_ARCH"
+    else
+        _arch="$XBPS_MACHINE"
+    fi
+
+    if [ "$pkgname" != "$sourcepkg" ]; then
+        if ! echo "$subpackages" | grep -q -w "$pkgname"; then
+            report_broken "${pkgname}-${version}_${revision}: this subpackage cannot be built for ${_arch}.\n"
         fi
+    fi
+
+    if [ -n "$archs" ]; then
         set -f
         for f in ${archs}; do
             set +f