[SV 63552] Change directories before constructing include paths

[SV 63552] Change directories before constructing include paths

* src/makeint.h (reset_makeflags): New function to handle changing
MAKEFLAGS from within makefiles.  Remove decode_env_switches().
* src/variable.c (set_special_var): Call reset_makeflags() instead
of various internal methods.
* src/main.c (decode_env_switches): Only internal now so make static.
(decode_switches): Don't invoke construct_include_path() yet.
(reset_makeflags): Decode env switches and construct include paths.
(main): Construct include paths after we process -C options.
* tests/scripts/options/dash-C: Rewrite to use new test constructs.
Add a test using both -C and -I together.
Add a test for multiple -C options.