wxMaxima should not rely on PWD being set

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -220,7 +220,11 @@ add_definitions(-DwxNO_UNSAFE_WXSTRING_CONV)
 if(WIN32 OR CYGWIN)
     file(RELATIVE_PATH RELPATH_TO_PO4A_PO_DIRECTORY "${CMAKE_BINARY_DIR}" "${CMAKE_SOURCE_DIR}/locales/manual")
 else()
-    file(RELATIVE_PATH RELPATH_TO_PO4A_PO_DIRECTORY "$ENV{PWD}" "${CMAKE_SOURCE_DIR}/locales/manual")
+    set(_pwd "$ENV{PWD}")
+    if(NOT _pwd OR NOT IS_ABSOLUTE "${_pwd}" OR NOT IS_DIRECTORY "${_pwd}")
+        set(_pwd "${CMAKE_BINARY_DIR}")
+    endif()
+    file(RELATIVE_PATH RELPATH_TO_PO4A_PO_DIRECTORY "${_pwd}" "${CMAKE_SOURCE_DIR}/locales/manual")
 endif()
 file(GLOB MANUAL_LANGUAGES "${CMAKE_CURRENT_SOURCE_DIR}/locales/manual/*.po")
 list(TRANSFORM MANUAL_LANGUAGES REPLACE ".*/(.*).po$" "\\1")
