Fix compile on Cygwin

[weiwangncar]

Allow compilation on Cygwin to complete

TYPE: bug fix

KEYWORDS: compilation, Cygwin,

SOURCE: Daniel Wesloh (Penn State)

DESCRIPTION OF CHANGES:
Problem:
Compiling WRF failed on Cygwin due to lack of netCDF4

Solution:
Match assumptions about USENETCDFPAR (#1743 would also fix this) and pass flags to allow legacy Fortran constructs (disallowed by default with GCC 10)

ISSUE: For use when this PR closes an issue.
Fixes #1271

LIST OF MODIFIED FILES:

M       configure
M       doc/README.cygwin.md
M       doc/README.netcdf4par

TESTS CONDUCTED:

1. Checked whether model compiles on Cygwin in Fix cygwin compile DWesl/WRF#1
2. The Jenkins tests have passed.

RELEASE NOTE: Fix compilation on Cygwin.

[weiwangncar]

The regression test results:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None

[weiwangncar]

@islas Need to re-approve... Thanks...