GenericProject: Do not ignore flags that are not in the first line

Now flags in all lines of the .cxxflags and the .cflags files are parsed. Not only the flags in the first lines of these files.

Change-Id: I2abddbfce9888dd9cb94aa75b562dc86997bdf48
Reviewed-by: hjk <hjk@qt.io>

src/plugins/genericprojectmanager/genericproject.cpp

@@ -406,7 +406,10 @@ static QStringList readFlags(const QString &filePath)
     const QStringList lines = readLines(filePath);
     if (lines.isEmpty())
         return QStringList();
-    return QtcProcess::splitArgs(lines.first());
+    QStringList flags;
+    for (const auto &line : lines)
+        flags.append(QtcProcess::splitArgs(line));
+    return flags;
 }
 
 void GenericBuildSystem::parse(RefreshOptions options)