Fix errors when eval path contains whitespace

This fixes the following error when running ./configure under cygwin:

The eval command dislikes the whitespace of my path unless I add the additional single quotes.

```
checking if you have a recent GNU tar... yes
./configure: eval: line 6188: syntax error near unexpected token `('
./configure: eval: line 6188: `echo /cygdrive/c/Program Files (x86)/Git/bin/openssl'
./configure: line 6188: warning: syntax errors in . or eval will cause future versions of the shell to abort as Posix requires
./configure: eval: line 6188: syntax error near unexpected token `('
./configure: eval: line 6188: `echo /cygdrive/c/Program Files (x86)/Git/bin/bison'
./configure: line 6188: warning: syntax errors in . or eval will cause future versions of the shell to abort as Posix requires
./configure: eval: line 6188: syntax error near unexpected token `('
./configure: eval: line 6188: `echo /cygdrive/c/Program Files (x86)/Git/bin/flex'
./configure: line 6188: warning: syntax errors in . or eval will cause future versions of the shell to abort as Posix requires
./configure: eval: line 6188: syntax error near unexpected token `('
./configure: eval: line 6188: `echo /cygdrive/c/Program Files (x86)/Git/bin/patch'
./configure: line 6188: warning: syntax errors in . or eval will cause future versions of the shell to abort as Posix requires
configure: creating ./config.status
config.status: creating config.h
config.status: config.h is unchanged
```