gallium: Expand libc check to be platform OS check
endianness on musl based systems can also be detected in same way as
glibc, therefore check for __linux__
define instead of __GLIBC__
Fixes build on musl/mips
| ../../../../git/src/gallium/include/pipe/p_config.h:171:2: error: #error Unknown Endianness
| #error Unknown Endianness
| ^~~~~
Edited by Gert Wollny