Tests of installing package using gfortran with flag --std=f2008.  Recent
gfortran defaults to F2018 with GNU extensions so this would catch usage of
F2018 features (but none were seen).  However, it also catches use of
'GNU extensions', many of which are actually long obsolete (pre-F77) functions.

These include
DFLOAT - use DBLE
DREAL - use REAL
IMAG - use AIMAG
DXMPLX - use CMPLX
DERFC - use ERFC for >= F2008

etime int8 getarg getpid are GNU extensions.
dlgama seems to be a GNU extension: the F2008 function is LOG_GAMMA.
isnan is a GNU extension.  There are standard ways to do this as from F2003,
or you can use if(my_var /= my_var).

DOUBLE COMPLEX and COMPLEX*16 are extensions but unavoidable.

etime can be replaced by the standard CPU_TIME and/or SYSTEM_CLOCK,
if actually used (in most, maybe all, cases it is not used in the package)..