cmake: Do not skip configure phase if CMakeCache.txt doesn't exist

I think there's a regression introduced by !254 (merged).

When building the following moduleset:

<cmake id="aom" cmakeargs="-DBUILD_SHARED_LIBS=1 -DENABLE_DOCS=OFF -DENABLE_EXAMPLES=OFF -DENABLE_TOOLS=OFF -DENABLE_TESTS=OFF">
    <pkg-config>libaom.pc</pkg-config>
    <branch module="aom"
            repo="aomedia.googlesource.com"
            version="3.8.0"
            tag="v3.8.0"
            checkoutdir="libaom-${version}">
    </branch>
</cmake>

I get this error:

* Checking out aom * [1/32]
Cloning into 'libaom-3.8.0'...
warning: templates not found in /home/pwuser/.git_template
Fetching origin
HEAD is now at b681eac839 Update CHANGELOG for libaom 3.8.0
git clone https://aomedia.googlesource.com/aom libaom-3.8.0 --branch v3.8.0
git remote set-url origin https://aomedia.googlesource.com/aom
git remote update origin
git checkout v3.8.0
* Applying patch fix-aom-build-arm32.patch * [1/32]
git apply /home/pwuser/webkit/Tools/jhbuild/patches/fix-aom-build-arm32.patch
* Building aom * [1/32]
Error during phase build of aom: [Errno 2] No such file or directory: '/home/pwuser/webkit/WebKitBuild/DependenciesWPE/Build/libaom-3.8.0'
ninja  
Failed to build WPE port dependencies with jhbuild
Died at ./Tools/Scripts/update-webkitwpe-libs line 26.

Debugging the code shows JHBuild is trying to build the freshly checkout module without running the configure step. The configure step is skipped because PHASE_CHECKOUT is missing in skip_configure.

cc @nbenitez