Trac #30643: cygwin-minimal, cygwin-standard: scipy build fails: 'u_int' has not been declared

https://github.com/mkoeppe/sage/runs/1153050152
{{{
In file included from /usr/include/sys/config.h:5,
                   from /usr/include/_ansi.h:11,
                   from /usr/include/sys/reent.h:13,
                   from /usr/include/math.h:5,
                   from /usr/lib/gcc/x86_64-pc-
cygwin/10/include/c++/cmath:45,
                   from scipy/spatial/ckdtree/src/query.cxx:1:
  /usr/include/sys/features.h:255: note: this is the location of the
previous definition
    255 | #define __BSD_VISIBLE  0
        |
  In file included from
/cygdrive/d/a/sage/sage/local/include/python3.8/pyport.h:219,
                   from
/cygdrive/d/a/sage/sage/local/include/python3.8/Python.h:63,
                   from /cygdrive/d/a/sage/sage/local/lib/python3.8
/site-packages/numpy/core/include/numpy/npy_common.h:11,
                   from scipy/spatial/ckdtree/src/ckdtree_decl.h:10,
                   from scipy/spatial/ckdtree/src/query.cxx:13:
  /usr/include/sys/time.h:106:34: error: 'u_int' has not been declared
    106 | bintime_mul(struct bintime *_bt, u_int _x)
        |                                  ^~~~~
  g++: scipy/spatial/ckdtree/src/build.cxx
}}}

Also seen in `cygwin-standard`
(https://github.com/sagemath/sage/runs/1190880811)

Blocker for 9.2 because it is a regression in platform support

URL: https://trac.sagemath.org/30643
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Erik Bray

Author: Release Manager

build/pkgs/numpy/spkg-install.in

@@ -11,6 +11,11 @@ else
     export LDFLAGS="${LDFLAGS} -shared"
 fi
 
+if [ "$UNAME" = "CYGWIN" ]; then
+    # Trac #30643
+    export CPPFLAGS="${CPPFLAGS} -D_GNU_SOURCE"
+fi
+
 sage-python23 ../lapack_conf.py
 
 # Make sure that the fortran objects are compiled with -fPIC

build/pkgs/scipy/spkg-install.in

@@ -18,6 +18,12 @@ fi
 # Make sure that the fortran objects are compiled with -fPIC
 export FFLAGS="$FFLAGS -fPIC"
 export FCFLAGS="$FCFLAGS -fPIC"
+
+if [ "$UNAME" = "CYGWIN" ]; then
+    # Trac #30643
+    export CPPFLAGS="${CPPFLAGS} -D_GNU_SOURCE"
+fi
+
 if [ "$UNAME" = "CYGWIN" -a "$SAGE_DEBUG" = "yes" ]; then
     # Needed for just one or two modules when compiling in debug mode
     # Otherwise the debug symbols create too many sections in the binary