homebrew-osgeo4mac: GDAL fails to build on High Sierra with CLTools installed

Just noting that this is a known issue that I’ve been running into: https://github.com/Homebrew/homebrew-core/issues/14418#issuecomment-324257556

For instance:

In file included from /private/tmp/gdal2-20170927-17481-1shd4hy/gdal-2.2.2/port/cpl_conv.h:35:
In file included from /private/tmp/gdal2-20170927-17481-1shd4hy/gdal-2.2.2/port/cpl_port.h:659:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/cmath:313:9: error: no member named 'signbit' in the global namespace
using ::signbit;
      ~~^```