os/mac/pkgconfig: supply default homebrew_sdkroot

Library/Homebrew/os/mac/pkgconfig/10.14/libcurl.pc

Library/Homebrew/os/mac/pkgconfig/10.14/libexslt.pc

Library/Homebrew/os/mac/pkgconfig/10.14/libxml-2.0.pc

Library/Homebrew/os/mac/pkgconfig/10.14/libxslt.pc

Library/Homebrew/os/mac/pkgconfig/10.14/sqlite3.pc

Library/Homebrew/os/mac/pkgconfig/10.14/uuid.pc

Library/Homebrew/os/mac/pkgconfig/10.14/zlib.pc

Library/Homebrew/os/mac/pkgconfig/10.15/libcurl.pc

Library/Homebrew/os/mac/pkgconfig/10.15/libexslt.pc

Library/Homebrew/os/mac/pkgconfig/10.15/libxml-2.0.pc

Library/Homebrew/os/mac/pkgconfig/10.15/libxslt.pc

Library/Homebrew/os/mac/pkgconfig/10.15/sqlite3.pc

Library/Homebrew/os/mac/pkgconfig/10.15/uuid.pc

Library/Homebrew/os/mac/pkgconfig/10.15/zlib.pc