In PG 14, the source file "float.h" has one include:
<math.h>
It is missing "c.h", which contains the definitions for PGDLLIMPORT,
float4, float8, etc.
I have the latest Windows installation from EDB.
I checked in doxygen and the include is missing there, too:
https://doxygen.postgresql.org/float_8h_source.html
Best,
Ian Campbell