cmake: fixing toolchain_parse_make_rule to correctly handle \ files

Fixes: #37637

toolchain_parse_make_rule() parses depfiles and converted `\\` to the
CMake list separator `;`.

However, gcc -M might create depfiles with windows path separator `\`
causing this conversion to fail, as a path like:
c:\...\zephyr\samples\drivers\led_ws2812\nrf52-bindings.h

resulting in a file list as:
c:;...;zephyr;samples;drivers;led_ws2812;nrf52-bindings.h

which results in a CMake configure dependency to be added to `C:`.
As C: is always newer than the build.ninja file, this resulted in
continues CMake re-invocation.

As a small side-note, the `\` in file name did only occur in situations
where a relative past had been used elsewhere in the build system, such
as here:
https://github.com/zephyrproject-rtos/zephyr/blob/\
c3050a5/samples/drivers/led_ws2812/\
boards/nrf52dk_nrf52832.overlay#L9

To ensure proper handling of files, then all files are converted to
CMake paths, that is with forward slashes: `/`

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

cmake/extensions.cmake

@@ -1797,30 +1797,35 @@ endfunction()
 # The argument 'include_files' is an output parameter with the result
 # of parsing the include files.
 function(toolchain_parse_make_rule input_file include_files)
-  file(READ ${input_file} input)
+  file(STRINGS ${input_file} input)
 
   # The file is formatted like this:
   # empty_file.o: misc/empty_file.c \
   # nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts \
   # nrf52840_qiaa.dtsi
 
-  # Get rid of the backslashes
-  string(REPLACE "\\" ";" input_as_list ${input})
+  # The dep file will contain `\` for line continuation.
+  # This results in `\;` which is then treated a the char `;` instead of
+  # the element separator, so let's get the pure `;` back.
+  string(REPLACE "\;" ";" input_as_list ${input})
 
   # Pop the first line and treat it specially
-  list(GET input_as_list 0 first_input_line)
+  list(POP_FRONT input_as_list first_input_line)
   string(FIND ${first_input_line} ": " index)
   math(EXPR j "${index} + 2")
   string(SUBSTRING ${first_input_line} ${j} -1 first_include_file)
-  list(REMOVE_AT input_as_list 0)
 
-  list(APPEND result ${first_include_file})
+  # Remove whitespace before and after filename and convert to CMake path.
+  string(STRIP "${first_include_file}" first_include_file)
+  file(TO_CMAKE_PATH "${first_include_file}" first_include_file)
+  set(result "${first_include_file}")
 
-  # Add the other lines
-  list(APPEND result ${input_as_list})
-
-  # Strip away the newlines and whitespaces
-  list(TRANSFORM result STRIP)
+  # Remove whitespace before and after filename and convert to CMake path.
+  foreach(file ${input_as_list})
+    string(STRIP "${file}" file)
+    file(TO_CMAKE_PATH "${file}" file)
+    list(APPEND result "${file}")
+  endforeach()
 
   set(${include_files} ${result} PARENT_SCOPE)
 endfunction()