BUG: Using CMake variable in external builds with autoconf environment

Previously, the external projects of FFTW, SWIG and PCRE were affected
by environmental variable such as CC, CXX, CFLAGS etc. The could be
tested by setting CXX=/dev/null in the environment. Additionally,
these setting could change from when cmake was first run to when the
external project was (re)configured and compiled.

Now, the environment variable which effect the autoconf are not set in
via the "env" command before configuration.

Change-Id: Ie2d4f43c10d40186062a2e2917eff26b03b9815c
Merge topic 'FFTWExternalInEnviroment'

da4ce96 BUG: Using CMake variable in external builds with autoconf environment