xip_ram_perms/CMakeLists.txt: unset environment variables CFLAGS, CXX…

[josch]

…FLAGS and LDFLAGS

If the user set these environment variables to influence the picotool build, unset them here so that they do not influence the pico-sdk build. This is especially required for flags that are not supported by arm-none-eabi compilers.

As suggested by @will-v-pi in #139 (comment)

[will-v-pi]

Could you target this at the develop branch rather than master?

Also, I think the same changes should probably be added the the picoboot_flash_id/CMakeLists.txt file as well, as that also uses a pico-sdk build

[lurch]

What if the user wants to set different CFLAGS for the picotool build and the xip_ram_perms build? Would it be worth supporting e.g. a PICO_SDK_CFLAGS setting, which would be used as CFLAGS when building xip_ram_perms and picoboot_flash_id ? 🤷 (Apologies in advance if this is a stupid question!)

[will-v-pi]

What if the user wants to set different CFLAGS for the picotool build and the xip_ram_perms build? Would it be worth supporting e.g. a PICO_SDK_CFLAGS setting, which would be used as CFLAGS when building xip_ram_perms and picoboot_flash_id ? 🤷 (Apologies in advance if this is a stupid question!)

I can’t really think of a use case for that, as the SDK sets all the relevant CFLAGS required to build correctly? Especially for a minimal binary like this, I’m not sure what extra CFLAGS might be required. So I think we can merge this as is, and revisit if anyone comes up with a need for a PICO_SDK_CFLAGS setting

[lurch]

I can’t really think of a use case for that, as the SDK sets all the relevant CFLAGS required to build correctly?

Fair enough, I guess I was just playing devil's advocate 😆