Cmake: Fix autodetection of cmake on Windows

By using some of the infrastructure in Utils::Environment. Due to
wanting to find all cmake executables in PATH and not just the
first one, we need some custom code.

Change-Id: If3161d712b9f6e659450a3d0647f83344e68ba5d
Task-number: QTCREATORBUG-14740
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Benjamin Zeller <benjamin.zeller@canonical.com>
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/libs/utils/environment.h

@@ -105,6 +105,7 @@ public:
     FileName searchInPath(const QString &executable,
                           const QStringList &additionalDirs = QStringList()) const;
     QStringList path() const;
+    QStringList appendExeExtensions(const QString &executable) const;
 
     QString expandVariables(const QString &input) const;
     QStringList expandVariables(const QStringList &input) const;