Peter:
setenv GPC_SDK /Volumes/SSD2/Macintosh_HD/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk
Note! This is the location on my system only; presumably nobody else has SSD2 ... Find the Xcode.app and then follow the path.
You can find it using: xcode-select -p find . -name "*SDK*" -print | grep MacOSX.platform
which can be done on one line:
cd `xcode-select -p`;find . -name "*SDK*" -print | grep MacOSX.platform
Then one has to select the directory that ends in '.sdk'.
Tom
Thomas D. Schneider, Ph.D. Senior Investigator National Institutes of Health National Cancer Institute Center for Cancer Research Gene Regulation and Chromosome Biology Laboratory Molecular Information Theory Group Frederick, Maryland 21702-1201 https://schneider.ncifcrf.gov (current link) https://alum.mit.edu/www/toms (permanent link)