Strip single quote from LDFLAGS and CPPFLAGS

To building 2.1.0-dev with ruby-build master happened build failure. It seems that LDFLAGS and CPPFLAGS contains wrong variables such as single quote.

PS: ruby-trunk checked these variables and stopped build process by https://github.com/ruby/ruby/commit/3636f8c0f56ddf15e26e28e7a38e748588fad976