um, Compiler warned, but test passed. It seems a only warning.

build log:
$ ./configure --prefix=/opt/py34
$ make
...
building 'readline' extension
gcc -Wno-unused-result -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -Werror=declaration-after-statement -I./Include -I/opt/py34/include -I. -IInclude -I/usr/local/include -I/home/masayuki/src/CPython-3.4/Include -I/home/masayuki/src/CPython-3.4 -c /cygdrive/d/cyghome/masayuki/src/CPython-3.4/Modules/readline.c -o build/temp.cygwin-1.7.35-i686-3.4/cygdrive/d/cyghome/masayuki/src/CPython-3.4/Modules/readline.o
/cygdrive/d/cyghome/masayuki/src/CPython-3.4/Modules/readline.c:1065:23: warning: '_PyOS_ReadlineTState' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
 extern PyThreadState* _PyOS_ReadlineTState;
                       ^
...

test log:
$ /opt/py34/bin/python3.4m.exe -E -Wd -mtest -v test_readline
== CPython 3.4.3+ (3.4:ce2b9f160391+, Apr 25 2015, 20:45:32) [GCC 4.9.2]
==   CYGWIN_NT-6.0-1.7.35-0.287-5-3-i686-32bit-WindowsPE little-endian
==   hash algorithm: siphash24 32bit
==   /tmp/test_python_2660
Testing with flags: sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=1, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0)
[1/1] test_readline
testHistoryUpdates (test.test_readline.TestHistoryManipulation) ... ok
test_init (test.test_readline.TestReadline) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.199s

OK
1 test OK.