cabal new-build fails to pass any options for `libexecsubdir` when using 'build-type: configure'.

Steps to reproduce:

```bash
$ cabal unpack old-time
$ cd old-time-1.1.0.3/
$ cabal new-build --with-ghc=ghc-8.0.2 --with-ghc-pkg=ghc-pkg-8.0.2
Resolving dependencies...
In order, the following will be built (use -v for more details):
- old-time-1.1.0.3 (lib) (first run)
Configuring library for old-time-1.1.0.3..
configure: error: unrecognized option: `--libexecsubdir='
Try `./configure --help' for more information
```

I am using head. Excerpt of output with -v:

```bash
...
/bin/sh ./configure '--with-compiler=ghc' '--prefix=/home/vanessa/.cabal' '--bindir=/home/vanessa/.cabal/bin' '--libdir=/home/vanessa/.cabal/lib/x86_64-linux-ghc-8.0.2/old-time-1.1.0.3-inplace' '--libexecdir=/home/vanessa/.cabal/libexec/x86_64-linux-ghc-8.0.2/old-time-1.1.0.3' '--libexecsubdir=' '--datadir=/home/vanessa/.cabal/share/x86_64-linux-ghc-8.0.2/old-time-1.1.0.3' '--sysconfdir=/home/vanessa/.cabal/etc' 'CC=/usr/bin/gcc'
configure: error: unrecognized option: `--libexecsubdir='
Try `./configure --help' for more information
```

Excerpt from my ~/.cabal/config:

```
...
install-dirs user
-- prefix: /home/vanessa/.cabal
-- bindir: $prefix/bin
-- libdir: $prefix/lib
-- libsubdir: $abi/$libname
-- dynlibdir: $libdir/$abi
-- libexecdir: $prefix/libexec
-- libexecsubdir: $abi/$pkgid
-- datadir: $prefix/share
-- datasubdir: $abi/$pkgid
-- docdir: $datadir/doc/$abi/$pkgid
-- htmldir: $docdir/html
-- haddockdir: $htmldir
-- sysconfdir: $prefix/etc

install-dirs global
-- prefix: /usr/local
-- bindir: $prefix/bin
-- libdir: $prefix/lib
-- libsubdir: $abi/$libname
-- dynlibdir: $libdir/$abi
-- libexecdir: $prefix/libexec
-- libexecsubdir: $abi/$pkgid
-- datadir: $prefix/share
-- datasubdir: $abi/$pkgid
-- docdir: $datadir/doc/$abi/$pkgid
-- htmldir: $docdir/html
-- haddockdir: $htmldir
-- sysconfdir: $prefix/etc
```