#23 modify makefile.spws for gcc 10

[seismick]

Under spws_home/util tree, there are dozens of Fortran 77 files
(with .f extension). With gfortran 10, some of these will no longer compile,
due to version 10 being stricter than old versions. And since more linux
distros now come with gcc 10, this needs to be overcome.
The compiling of the spws_home/util F77 files is governed by makefile.spws at
around line 906. Adding -std=legacy will avoid the strict compiler
errors. A conditional is needed like this:

ifeq "$(GCC_GT9)" "1"
$(FC) -std=legacy -o $.o $(SRCDIR)/$.f
else
$(FC) -o $.o $(SRCDIR)/$.f
endif

See feature requests 17 and 19 for the variable GCC_GT9. It is defined in
setup_cpseis_environment.sh in the latest revisions.