Prevent cmake error when prefix empty (pybind#1986)

Author: mattdawkins

tools/FindPythonLibsNew.cmake

@@ -140,9 +140,9 @@ list(GET _PYTHON_VERSION_LIST 1 PYTHON_VERSION_MINOR)
 list(GET _PYTHON_VERSION_LIST 2 PYTHON_VERSION_PATCH)
 
 # Make sure all directory separators are '/'
-string(REGEX REPLACE "\\\\" "/" PYTHON_PREFIX ${PYTHON_PREFIX})
-string(REGEX REPLACE "\\\\" "/" PYTHON_INCLUDE_DIR ${PYTHON_INCLUDE_DIR})
-string(REGEX REPLACE "\\\\" "/" PYTHON_SITE_PACKAGES ${PYTHON_SITE_PACKAGES})
+string(REGEX REPLACE "\\\\" "/" PYTHON_PREFIX "${PYTHON_PREFIX}")
+string(REGEX REPLACE "\\\\" "/" PYTHON_INCLUDE_DIR "${PYTHON_INCLUDE_DIR}")
+string(REGEX REPLACE "\\\\" "/" PYTHON_SITE_PACKAGES "${PYTHON_SITE_PACKAGES}")
 
 if(CMAKE_HOST_WIN32 AND NOT (MSYS OR MINGW))
     set(PYTHON_LIBRARY