Replace nonstandard isnan() call with value input checks instead (#2061)

TYPE: bug fix

KEYWORDS: compilation, nonstandard

SOURCE: internal

DESCRIPTION OF CHANGES:
Problem:
PR #1944 introduced a nonstandard function call `isnan()` which breaks
compilation for compilers which don't support this extension.

Solution:
A potential alternative is to use the `ieee_is_nan()` call from
`ieee_arithmetic`. As there is conditional compilation of this module
which requires Fortran 2003 standard, an easier alternative is just to
check the input values before calling `acos()`

phys/module_wind_mav.F

@@ -1230,10 +1230,11 @@ function find_turb(xc, yc, xt, yt, u, v, sr_angle, sr_dis)  result(ft)
           tmp2 = sign(tmp1,tmp2)
       end if
 
-      angle = real(acos(tmp1/tmp2), kind = 4)
-
-      if (isnan(angle)) then
-         angle = 0.
+      tmp1 = tmp1 / tmp2
+      if (tmp1 < -1.0_8 .or. tmp1 > 1.0_8) then
+          angle = 0.
+      else
+          angle = real(acos(tmp1), kind = 4)
       end if
 
       if (abs(angle) <= sr_angle) then