UMRead C library: `make` warnings with Clang compiler

The CI job using MacOS and its default compiler Clang is currently raising some warnings on building the UMRead C library, all amounting to unwgdos.c:371:45: warning: shifting a negative signed value is undefined [-Wshift-negative-value], with full make output of (or see also https://github.com/NCAS-CMS/cf-python/runs/5130889501?check_suite_focus=true for the job log, etc.):

$ cd /Users/runner/work/cf-python/cf-python/main/cf/umread_lib/c-lib
$ make
clang -Wall -fPIC -DOSX -c umfile.c
clang -Wall -fPIC -DOSX -c error.c
clang -Wall -fPIC -DOSX -c filetype.c
clang -Wall -fPIC -DOSX -c malloc.c
clang -Wall -fPIC -DOSX -c linklist.c
clang -Wall -fPIC -DOSX -c new_structs.c
clang -Wall -fPIC -DOSX -c swap.c
/Applications/Xcode_13.2.1.app/Contents/Developer/usr/bin/make -C type-dep
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o umfile_test_typedep_sgl.o umfile_test_typedep.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o interpret_header_sgl.o interpret_header.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o read_sgl.o read.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o process_vars_sgl.o process_vars.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o debug_dump_sgl.o debug_dump.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o date_and_time_sgl.o date_and_time.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o compare_sgl.o compare.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o levels_sgl.o levels.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o axes_sgl.o axes.c
clang -Wall -fPIC -DOSX -I.. -c -DSINGLE -o unwgdos_sgl.o unwgdos.c
unwgdos.c:371:45: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
      *inum = (*icomp << (31-istart)) & (~0 << 31);
                                         ~~ ^
unwgdos.c:3[9](https://github.com/NCAS-CMS/cf-python/runs/5130889501?check_suite_focus=true#step:10:9)6:[27](https://github.com/NCAS-CMS/cf-python/runs/5130889501?check_suite_focus=true#step:10:27): warning: shifting a negative signed value is undefined [-Wshift-negative-value]
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o umfile_test_typedep_dbl.o umfile_test_typedep.c
  i = (ui >> ibit) & ~(~0 << 1);
                       ~~ ^
2 warnings generated.
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o interpret_header_dbl.o interpret_header.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o read_dbl.o read.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o process_vars_dbl.o process_vars.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o debug_dump_dbl.o debug_dump.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o date_and_time_dbl.o date_and_time.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o compare_dbl.o compare.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o levels_dbl.o levels.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o axes_dbl.o axes.c
clang -Wall -fPIC -DOSX -I.. -c -DDOUBLE -o unwgdos_dbl.o unwgdos.c
unwgdos.c:371:45: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
      *inum = (*icomp << ([31](https://github.com/NCAS-CMS/cf-python/runs/5130889501?check_suite_focus=true#step:10:31)-istart)) & (~0 << 31);
                                         ~~ ^
rm -f umfile_typedep.a
unwgdos.c:[39](https://github.com/NCAS-CMS/cf-python/runs/5130889501?check_suite_focus=true#step:10:39)6:27: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
ar r umfile_typedep.a  umfile_test_typedep_sgl.o  interpret_header_sgl.o  read_sgl.o  process_vars_sgl.o  debug_dump_sgl.o  date_and_time_sgl.o  compare_sgl.o  levels_sgl.o  axes_sgl.o  unwgdos_sgl.o  umfile_test_typedep_dbl.o  interpret_header_dbl.o  read_dbl.o  process_vars_dbl.o  debug_dump_dbl.o  date_and_time_dbl.o  compare_dbl.o  levels_dbl.o  axes_dbl.o  unwgdos_dbl.o
  i = (ui >> ibit) & ~(~0 << 1);
                       ~~ ^
2 warnings generated.
ar: creating archive umfile_typedep.a
clang -dynamiclib -o umfile.so umfile.o error.o filetype.o malloc.o linklist.o new_structs.o swap.o -force_load type-dep/umfile_typedep.a

@alaniwi please could you look at this and let us know if it's an issue or not (I don't know enough C to be sure myself, and David is away until 28th)? Thanks.

Start in cf/umread_lib/c-lib/unwgdos.c at the reported lines 371 and 396, then run make with the Clang configuration shown in the issue. Check the compiler documentation or project context to determine whether the shifts are valid; done means the macOS build no longer emits these warnings without changing library behavior.