* libdrm: fix one configure error and require libpciaccess
Failure with `LIBS`: the linker can't find `-lrt` so configure fails on
darwin-bigsur %apple-clang@12.0.5
```
>> 22 configure: error: in `/private/var/folders/gy/mrg1ffts2h945qj9k29s1l1dvvmbqb/T/s3j/spack-s
tage/spack-stage-libdrm-2.4.100-ofhk6m25n2pi427ihnxmvjkfmgyzlrqc/spack-src':
>> 23 configure: error: C compiler cannot create executables
24 See `config.log' for more details
See build log for details:
/var/folders/gy/mrg1ffts2h945qj9k29s1l1dvvmbqb/T/s3j/spack-stage/spack-stage-libdrm-2.4.100-ofhk6m25n2pi427ihnxmvjkfmgyzlrqc/spack-build-out.txt
```
* libpciaccess: Mark conflict with darwin
```
make[2]: *** [common_init.lo] Error 1
make[2]: *** Waiting for unfinished jobs....
common_interface.c:75:10: fatal error: 'sys/endian.h' file not found
^~~~~~~~~~~~~~
```
and
```
common_init.c:73:3: error: "Unsupported OS"
```
and others
1bf84d170f