Add option to use clang's MSVC ABI

[Oliver-makes-code]

Fixes #728.

Adds QJS_CLANG_MSVC_ABI option, defaulted to OFF. It doesn't link libm if it's ON.
I opted to have it be an option, because implementing behavior to check the clang target would add too much complexity.

[saghul]

Can't we detect this in CMake?

[Oliver-makes-code]

You can detect it, but since it's a generally niche use-case, it didn't seem worth the extra complexity to me.

[saghul]

I politely disagree :-)

Let's see how much it really takes to get this right.

Would this work?

find_library(M_LIBRARIES m)
if(M_LIBRARIES)
    list(APPEND qjs_libs m)
endif()

[bnoordhuis]

it's a generally niche use-case, it didn't seem worth the extra complexity to me.

The counterargument is that build options are really hard to remove once people start depending on them, so adding them should be done sparingly.

[Oliver-makes-code]

I politely disagree :-)

Let's see how much it really takes to get this right.

Would this work?

find_library(M_LIBRARIES m)
if(M_LIBRARIES)
    list(APPEND qjs_libs m)
endif()

This could work

[saghul]

Can you give it a try? If it works we could just land that.

[Oliver-makes-code]

It seems to work