build error "readline not found"

[nyuichi]

According to @SaitoAtsushi, while building Picrin scheme on Windows, cmake warns that readline module is not found, although picrin should drop readline support when there's not libreadline on the environment at compile time.

-- The C compiler identification is GNU 4.8.1
-- The CXX compiler identification is GNU 4.8.1
-- Check for working C compiler: C:/MinGW/bin/cc.exe
-- Check for working C compiler: C:/MinGW/bin/cc.exe -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Check for working CXX compiler: C:/MinGW/bin/c++.exe
-- Check for working CXX compiler: C:/MinGW/bin/c++.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Found Git: c:/Program Files/Git/cmd/git.cmd (found version "1.7.11.msysgit.0")
-- Found FLEX: c:/MinGW/msys/1.0/bin/flex.exe (found version "2.5.35")
-- Found PkgConfig: c:/MinGW/msys/1.0/local/bin/pkg-config.exe (found version "0.28")
-- Could NOT find readline (missing:  READLINE_LIBRARY READLINE_INCLUDE_DIR)
CMake Error: The following variables are used in this project, but they are set
to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
READLINE_LIBRARY
    linked by target "repl" in directory C:/home/picrin

-- Configuring incomplete, errors occurred!
See also "C:/home/picrin/build/CMakeFiles/CMakeOutput.log".
See also "C:/home/picrin/build/CMakeFiles/CMakeError.log".

cf. https://twitter.com/SaitoAtsushi/status/448403948425064448, https://twitter.com/SaitoAtsushi/status/448464881923006464