Under Cygwin 64 and 32, malloca compile fails because gl/stdint.h
is generated (for some reason) with no value set for 
GNULIBHEADERS_OVERRIDE_WINT_T in #if, as no tests are configured
to set the value:

compile.log:
...
  CC       malloca.lo
In file included from /.../src/cygwin/libidn/libidn-1.37-1.x86_64/src/libidn-1.37/gl/malloca.h:24,
                 from /.../src/cygwin/libidn/libidn-1.37-1.x86_64/src/libidn-1.37/gl/malloca.c:22:
./stdint.h:89:5: error: #if with no expression
   89 | #if
      |     ^
make[4]: *** [Makefile:1560: malloca.lo] Error 1
make[4]: Leaving directory '/.../src/cygwin/libidn/libidn-1.37-1.x86_64/build/gl'
make[3]: *** [Makefile:1579: all-recursive] Error 1
make[3]: Leaving directory '/.../src/cygwin/libidn/libidn-1.37-1.x86_64/build/gl'
make[2]: *** [Makefile:1455: all] Error 2
make[2]: Leaving directory '/.../src/cygwin/libidn/libidn-1.37-1.x86_64/build/gl'
make[1]: *** [Makefile:1417: all-recursive] Error 1
make[1]: Leaving directory '/.../src/cygwin/libidn/libidn-1.37-1.x86_64/build'
make: *** [Makefile:1343: all] Error 2
*** ERROR: make failed


build/gl/stdint.h:
...
/* Override WINT_MIN and WINT_MAX if gnulib's <wchar.h> or <wctype.h> overrides
   wint_t.  */
#if 
# undef WINT_MIN
# undef WINT_MAX
# define WINT_MIN 0x0U
# define WINT_MAX 0xffffffffU
#endif
...


src/libidn-1.37/gl/stdint.in.h:
...
/* Override WINT_MIN and WINT_MAX if gnulib's <wchar.h> or <wctype.h> overrides
   wint_t.  */
#if @GNULIBHEADERS_OVERRIDE_WINT_T@
# undef WINT_MIN
# undef WINT_MAX
# define WINT_MIN 0x0U
# define WINT_MAX 0xffffffffU
#endif
...


build/gl/Makefile:
...
# We need the following in order to create <stdint.h> when the system
# doesn't have one that works with the given compiler.
stdint.h: stdint.in.h $(top_builddir)/config.status
	$(AM_V_GEN)rm -f $@-t $@ && \
	{ echo '/* DO NOT EDIT! GENERATED AUTOMATICALLY! */'; \
	  sed -e 's|@''GUARD_PREFIX''@|GL_LGL|g' \
	      -e 's/@''HAVE_STDINT_H''@/$(HAVE_STDINT_H)/g' \
	      -e 's|@''INCLUDE_NEXT''@|$(INCLUDE_NEXT)|g' \
	      -e 's|@''PRAGMA_SYSTEM_HEADER''@|#pragma GCC system_header|g' \
	      -e 's|@''PRAGMA_COLUMNS''@||g' \
	      -e 's|@''NEXT_STDINT_H''@|$(NEXT_STDINT_H)|g' \
	      -e 's/@''HAVE_C99_STDINT_H''@/$(HAVE_C99_STDINT_H)/g' \
	      -e 's/@''HAVE_SYS_TYPES_H''@/$(HAVE_SYS_TYPES_H)/g' \
	      -e 's/@''HAVE_INTTYPES_H''@/$(HAVE_INTTYPES_H)/g' \
	      -e 's/@''HAVE_SYS_INTTYPES_H''@/$(HAVE_SYS_INTTYPES_H)/g' \
	      -e 's/@''HAVE_SYS_BITYPES_H''@/$(HAVE_SYS_BITYPES_H)/g' \
	      -e 's/@''HAVE_WCHAR_H''@/$(HAVE_WCHAR_H)/g' \
	      -e 's/@''APPLE_UNIVERSAL_BUILD''@/$(APPLE_UNIVERSAL_BUILD)/g' \
	      -e 's/@''BITSIZEOF_PTRDIFF_T''@/$(BITSIZEOF_PTRDIFF_T)/g' \
	      -e 's/@''PTRDIFF_T_SUFFIX''@/$(PTRDIFF_T_SUFFIX)/g' \
	      -e 's/@''BITSIZEOF_SIG_ATOMIC_T''@/$(BITSIZEOF_SIG_ATOMIC_T)/g' \
	      -e 's/@''HAVE_SIGNED_SIG_ATOMIC_T''@/$(HAVE_SIGNED_SIG_ATOMIC_T)/g' \
	      -e 's/@''SIG_ATOMIC_T_SUFFIX''@/$(SIG_ATOMIC_T_SUFFIX)/g' \
	      -e 's/@''BITSIZEOF_SIZE_T''@/$(BITSIZEOF_SIZE_T)/g' \
	      -e 's/@''SIZE_T_SUFFIX''@/$(SIZE_T_SUFFIX)/g' \
	      -e 's/@''BITSIZEOF_WCHAR_T''@/$(BITSIZEOF_WCHAR_T)/g' \
	      -e 's/@''HAVE_SIGNED_WCHAR_T''@/$(HAVE_SIGNED_WCHAR_T)/g' \
	      -e 's/@''WCHAR_T_SUFFIX''@/$(WCHAR_T_SUFFIX)/g' \
	      -e 's/@''BITSIZEOF_WINT_T''@/$(BITSIZEOF_WINT_T)/g' \
	      -e 's/@''HAVE_SIGNED_WINT_T''@/$(HAVE_SIGNED_WINT_T)/g' \
	      -e 's/@''WINT_T_SUFFIX''@/$(WINT_T_SUFFIX)/g' \
	      -e 's/@''GNULIBHEADERS_OVERRIDE_WINT_T''@/$(GNULIBHEADERS_OVERRIDE_WINT_T)/g' \
	      < $(srcdir)/stdint.in.h; \
	} > $@-t && \
	mv $@-t $@
#stdint.h: $(top_builddir)/config.status
#	rm -f $@
...