Minor flaw in etc/config.sh/FFTW leads to always assuming it's not system-installed

I was writing and testing detailed installation steps for Ubuntu 16.04, when I stumbled upon two issues while building:

1. There is a typo in the variable checked, where FFTW_ARCH_PATH_PATH has one too many "_PATH" ;) Patch after the list below.

2. ThirdParty-*/makeFFTW was missing -f when calling unset, which @mark has already solved in Development/ThirdParty-plus#2.

The patch for fixing etc/config.sh/FFTW:

diff --git a/etc/config.sh/FFTW b/etc/config.sh/FFTW
index 7c0a488..f3cb4ea 100644
--- a/etc/config.sh/FFTW
+++ b/etc/config.sh/FFTW
@@ -64,7 +64,7 @@ then
     # it is either located within ThirdParty, or a central installation
     # outside of ThirdParty and must be added to the lib-path.
 
-    ending="${FFTW_ARCH_PATH_PATH##*-}"
+    ending="${FFTW_ARCH_PATH##*-}"
     if [ "$ending" != none -a "$ending" != system ]
     then
         _foamAddLib $FFTW_ARCH_PATH/lib$WM_COMPILER_LIB_ARCH