Build scripts: Fix path format for cmake calls

Make sure to pass posix style paths to CMAKE_PREFIX_PATH,
CMAKE_MODULE_PATH and CMAKE_INSTALL_PREFIX. Otherwise this can lead to
funny "Unknown control sequence \U" kind of errors.

Follow-up of fceaff1a6f

Change-Id: I1c8445f9c298a17115bca6b42f099a99e33d6de2
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

scripts/common.py

@@ -40,6 +40,12 @@ def is_linux_platform():
 def is_mac_platform():
     return sys.platform.startswith('darwin')
 
+def to_posix_path(path):
+    if is_windows_platform():
+        # should switch to pathlib from python3
+        return path.replace('\\', '/')
+    return path
+
 def check_print_call(command, workdir, env=None):
     print('------------------------------------------')
     print('COMMAND:')