allow using some libs as system provided (work around mssql linking vs system openssl)

src/SPC/builder/Extension.php

@@ -96,7 +96,8 @@ class Extension
             fn ($x) => $x->getStaticLibFiles(),
             $this->getLibraryDependencies(recursive: true)
         );
-        return implode(' ', $ret);
+        $libs = implode(' ', $ret);
+        return deduplicate_flags($libs);
     }
 
     /**

src/SPC/builder/LibraryBase.php

@@ -365,6 +365,24 @@ abstract class LibraryBase
 
     protected function isLibraryInstalled(): bool
     {
+        if ($pkg_configs = Config::getLib(static::NAME, 'pkg-configs', [])) {
+            $pkg_config_path = getenv('PKG_CONFIG_PATH') ?: '';
+            $search_paths = array_unique(array_filter(explode(is_unix() ? ':' : ';', $pkg_config_path)));
+
+            foreach ($pkg_configs as $name) {
+                $found = false;
+                foreach ($search_paths as $path) {
+                    if (file_exists($path . "/{$name}.pc")) {
+                        $found = true;
+                        break;
+                    }
+                }
+                if (!$found) {
+                    return false;
+                }
+            }
+            return true; // allow using system dependencies if pkg_config_path is explicitly defined
+        }
         foreach (Config::getLib(static::NAME, 'static-libs', []) as $name) {
             if (!file_exists(BUILD_LIB_PATH . "/{$name}")) {
                 return false;