gh-41104: never append ':' to the end of PKG_CONFIG_PATH in .homebrew-build-env

if `PKG_CONFIG_PATH` is not set, then `.homebrew-build-env` adds a
spurious `:` at the end. We fix this here.

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- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
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### ⌛ Dependencies

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URL: #41104
Reported by: Dima Pasechnik
Reviewer(s): David Coudert

Author: Release Manager

.homebrew-build-env

@@ -8,7 +8,11 @@ for l in bzip2 texinfo polymake; do
     fi
 done
 export PATH
-PKG_CONFIG_PATH="$HOMEBREW/lib/pkgconfig:$PKG_CONFIG_PATH"
+if [ -z "$PKG_CONFIG_PATH" ]; then
+    PKG_CONFIG_PATH="$HOMEBREW/lib/pkgconfig"
+else
+    PKG_CONFIG_PATH="$HOMEBREW/lib/pkgconfig:$PKG_CONFIG_PATH"
+fi
 # libpng.pc depends on zlib.pc
 for l in openblas openssl readline sqlite zlib; do
     if [ -d "$HOMEBREW/opt/$l/lib/pkgconfig" ]; then

build/pkgs/configure/checksums.ini

@@ -1,3 +1,3 @@
 tarball=configure-VERSION.tar.gz
-sha1=22617a1a12a247801d8cbf658f7df9978034e1a9
-sha256=fae518a4a51179e16965cbc55eb40a9b6ee89db7d81b5f7d9d0466262611b8a1
+sha1=351d5784c8d94f333ad5c9e66943dfce02a101a1
+sha256=72f00f33aea96f11cbf53696e58845875059c306e50dd111b1ce9399abc3b230

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-ad729a3fe96044ea67fc1f78ea2e0bf2706f29eb
+f186eec17539fa2c1f23ca75f06e6b5c0c08d60d