[build][cmake] Fix cmake with custom assembler

[terrelln]

Fixes cmake build when the C compiler isn't used as the assembler. We need to explicitly set the flag -x assembler-with-cpp to enable the preprocessor.

Fixes #3193.

[afbjorklund]

I think the -x flags is ignored by as, and it is still sending the rest of the cc flags (like before).

Fatal error: bad defsym; format is --defsym name=value

The needed fix is to make cmake use CMAKE_C_COMPILER instead of CMAKE_ASM_COMPILER

Since it is currently passing all the configured C flags, then it needs to be sent to a C compiler ?

[afbjorklund]

The default configuration when you run cmake, is to set the ASM compiler to "gcc"

But in Buildroot config, they explicitly define the ASM (cross) compiler to be "as"

-DCMAKE_ASM_COMPILER="/usr/bin/as" -DCMAKE_C_COMPILER="/usr/bin/gcc" -DCMAKE_CXX_COMPILER="/usr/bin/g++"

Which "makes sense", I suppose, but breaks the assumption that .S files are handled by cc

[terrelln]

Good point, I had only tested on my Mac. This now passes on both my Mac and Linux machines with -DCMAKE_ASM_COMPILER=as.

We just set the source language to C. While this isn't ideal, it looks like CMake is preserving this behavior for backwards compatibility.

[rkitover]

I can confirm that this works now on my Gentoo system as well, thank you!