R-ps fails to build on Tiger: macos.c: fatal error: libproc.h: No such file or directory #include <libproc.h>

[barracuda156]

/opt/local/bin/gcc-mp-7 -I"/opt/local/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -isystem/opt/local/include/LegacySupport -I/opt/local/include   -fPIC  -pipe -Os -arch ppc  -c api-posix.c -o api-posix.o
/opt/local/bin/gcc-mp-7 -I"/opt/local/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -isystem/opt/local/include/LegacySupport -I/opt/local/include   -fPIC  -pipe -Os -arch ppc  -c macos.c -o macos.o
macos.c:14:10: fatal error: libproc.h: No such file or directory
 #include <libproc.h>
          ^~~~~~~~~~~
compilation terminated.
make: *** [macos.o] Error 1
ERROR: compilation failed for package ‘ps’