FindBISON: Fix CMP0088 NEW behavior for non-absolute input paths

Now that the working directory is in the build tree, relative input
paths must be converted to an absolute path to remain relative to the
source directory.

Fixes: #19472

Help/release/3.14.rst

@@ -420,3 +420,11 @@ Changes made since CMake 3.14.0 include the following.
   from explicit use via :command:`include_directories` and
   :command:`target_include_directories` as they were in CMake 3.14.0
   through 3.14.4.
+
+3.14.6
+------
+
+* In CMake 3.14.0 through 3.14.5, the :module:`FindBISON` module
+  policy :policy:`CMP0088` ``NEW`` behavior accidentally interpreted
+  a relative path to the ``.y`` input as relative to the build tree
+  directory instead of the source tree directory.  This has been fixed.

Modules/FindBISON.cmake

@@ -257,15 +257,19 @@ if(BISON_EXECUTABLE)
         PARENT_SCOPE # undocumented, do not use outside of CMake
         )
       set(_BISON_WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+      set(_BisonInput "${BisonInput}")
       if("x${_BISON_CMP0088}x" STREQUAL "xNEWx")
         set(_BISON_WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+        if(NOT IS_ABSOLUTE "${_BisonInput}")
+          set(_BisonInput "${CMAKE_CURRENT_SOURCE_DIR}/${_BisonInput}")
+        endif()
       endif()
       unset(_BISON_CMP0088)
 
       add_custom_command(OUTPUT ${BISON_TARGET_outputs}
-        COMMAND ${BISON_EXECUTABLE} ${BISON_TARGET_cmdopt} -o ${BisonOutput} ${BisonInput}
+        COMMAND ${BISON_EXECUTABLE} ${BISON_TARGET_cmdopt} -o ${BisonOutput} ${_BisonInput}
         VERBATIM
-        DEPENDS ${BisonInput}
+        DEPENDS ${_BisonInput}
         COMMENT "[BISON][${Name}] Building parser with bison ${BISON_VERSION}"
         WORKING_DIRECTORY ${_BISON_WORKING_DIRECTORY})
 
@@ -273,12 +277,14 @@ if(BISON_EXECUTABLE)
 
       # define target variables
       set(BISON_${Name}_DEFINED TRUE)
-      set(BISON_${Name}_INPUT ${BisonInput})
+      set(BISON_${Name}_INPUT ${_BisonInput})
       set(BISON_${Name}_OUTPUTS ${BISON_TARGET_outputs} ${BISON_TARGET_extraoutputs})
       set(BISON_${Name}_COMPILE_FLAGS ${BISON_TARGET_cmdopt})
       set(BISON_${Name}_OUTPUT_SOURCE "${BisonOutput}")
       set(BISON_${Name}_OUTPUT_HEADER "${BISON_TARGET_output_header}")
 
+      unset(_BisonInput)
+
     endif()
   endmacro()
   #