Add ability to use Environment variable as default value in PARMV

[sasutils]

Allow use of an Environment variable as source of default value for parameter being checked by %PARMV() macro.

Example:

1 options set=STUDY=XYX101 ;
2 %let study=;
3 %parmv(study,_envar=STUDY);
4 %put &=study;
STUDY=XYX101

macros/parmv.sas

Line 1 in e8ab56b

%macro parmv