what is GMPXX, and is it ever used in Windows?

[CR20]

Hi,

GMPXX_INCLUDE_DIR-NOTFOUND
GMPXX_LIBRARIES-NOTFOUND

...during a CGAL convex hull app configuration in CMAKE--and yet those 
GMPXX header files are visible to me inside the CGAL directory:

gmpxx.h
   C:\dev\CGAL-5.1\include\CGAL
GMPXX_arithmetic_kernel.h
C:\dev\CGAL-5.1\include\CGAL
gmpxx_coercion_traits.h
C:\dev\CGAL-5.1\include\CGAL

I don't know CMAKE and CGAL beyond my downloading them shortly before 
this e-mail, but I managed to get Convex_hull_3 configured on CMAKE as 
well as generated.  (I left the "CGAL_WITH_GMPXX" checkbox unticked in 
CMAKE.)  I successfully compiled the project immediately after that.

What is GMPXX?  I'm finding little if any literature on it on the 
internet.  However 
https://doc.cgal.org/latest/Manual/configurationvariables.html says 
"Under Linux, the GMPXX is also searched for,...."  Does that imply that 
GMPXX is never searched for in CGAL for Windows (or anything for Windows 
at all for that matter)?

Thank you in advance for any assistance/guidance.