cmake: resolve symlinks when comparing source directories.

Testcase: trying to use an existing build dir which was
made for /d/foo where /d is a symlink to $HOME/d.

To help understanding the problem, the error message now prints
both source dirs, the one found in the builddir and the one we expected.

Change-Id: I9a24fbd6ef3cf8108c5edba0d2a5b99ecdea0e89
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/utils/fileutils.cpp

@@ -207,9 +207,8 @@ bool FileUtils::isFileNewerThan(const FileName &filePath, const QDateTime &timeS
 }
 
 /*!
-  Recursively resolves possibly present symlinks in \a filePath.
-  Unlike QFileInfo::canonicalFilePath(), this function will still return the expected target file
-  even if the symlink is dangling.
+  Recursively resolves symlinks if \a filePath is a symlink.
+  To resolve symlinks anywhere in the path, see canonicalPath
 
   \note Maximum recursion depth == 16.
 
@@ -226,6 +225,21 @@ FileName FileUtils::resolveSymlinks(const FileName &path)
     return FileName::fromString(f.filePath());
 }
 
+/*!
+  Recursively resolves possibly present symlinks in \a filePath.
+  Unlike QFileInfo::canonicalFilePath(), this function will not return an empty
+  string if path doesn't exist.
+
+  Returns the canonical path.
+*/
+FileName FileUtils::canonicalPath(const FileName &path)
+{
+    const QString result = QFileInfo(path.toString()).canonicalFilePath();
+    if (result.isEmpty())
+        return path;
+    return FileName::fromString(result);
+}
+
 /*!
   Like QDir::toNativeSeparators(), but use prefix '~' instead of $HOME on unix systems when an
   absolute path is given.