Add ability to pass additional options to clang via environnement variable.
With help from https://github.com/void-linux/void-packages/pull/26962

--- sources/shiboken2/ApiExtractor/clangparser/compilersupport.cpp
+++ sources/shiboken2/ApiExtractor/clangparser/compilersupport.cpp
@@ -305,6 +305,15 @@
 QByteArrayList emulatedCompilerOptions()
 {
     QByteArrayList result;
+
+    QByteArray shiboken_clang_options = qgetenv("SHIBOKEN_CLANG_OPTIONS");
+    // split to avoid double quoting drived by space inside it
+    if (!shiboken_clang_options.isEmpty()) {
+      for (const QByteArray &item: shiboken_clang_options.split(' ')) {
+        result.append(item);
+      }
+    }
+
 #if defined(Q_CC_MSVC)
     HeaderPaths headerPaths;
     result.append(QByteArrayLiteral("-fms-compatibility-version=19"));