Fix compiler error when building static libraries

[ghost]

No description provided.

[cahirwpz]

Umm... how to reproduce a bug to show this patch is necessary ?

[ghost]

When using m68k-amigaos-ar neither ixemul nor libnix linking will be used, and both switch are turned off.

Here is an example of such a usage:

git clone -b master https://github.com/llsth/amath.git
sh configure -m68000 m68k-amigaos- -noixemul
make

[cahirwpz]

When you link a static library neither libnix nor ixemul are used. This option is taken into account only when you link an executable file. When I modified configure script and added -noixemul to CC variable the build process went ok. Please consider if that's something you want.

[ghost]

@cahirwpz There is no reason to specify either options if all you want to do is to create a static library from code which includes stddef.h. Otherwise a static library produced this way would require to be linked to either libnix or ixemul.