build: set -U__STRICT_ANSI__ for newlib

This is (erroneously) required to enable various things in the
newlib headers.  As cygwin uses newlib, it is covered by this.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -2832,7 +2832,6 @@ case $target_os in
         SHFLAGS='-shared -Wl,--out-implib,$(SUBDIR)lib$(FULLNAME).dll.a'
         objformat="win32"
         enable dos_paths
-        add_cppflags -U__STRICT_ANSI__
         ;;
     *-dos|freedos|opendos)
         network_extralibs="-lsocket"
@@ -2933,6 +2932,7 @@ elif check_header _mingw.h; then
         die "ERROR: MinGW runtime version must be >= 3.15."
 elif check_cpp_condition newlib.h "defined _NEWLIB_VERSION"; then
     libc_type=newlib
+    add_cppflags -U__STRICT_ANSI__
 elif check_func_headers stdlib.h _get_doserrno; then
     libc_type=msvcrt
     add_compat strtod.o strtod=avpriv_strtod