Updates to FindFFTW.cmake and Findtcmalloc.cmake (AtlasLCG-00-00-67)

	* Modified FindFFTW.cmake to (hopefully) be able to cooperate
	  with the FFTW version built by External/AtlasFFTW.
	* Updated the code to be able to find/use tcmalloc from inside
	  of the LCG release. In case we don't build our own version
	  of it inside AtlasExternals.
	* Tagging as AtlasLCG-00-00-67

2016-08-19 Attila Krasznahorkay <Attila.Krasznahorkay@cern.ch>
	* Taught the code to recognise absolute path names in the LCG
	  text files. To be able to use the LCG nightly builds correctly
	  as well.
	* Tagging as AtlasLCG-00-00-66

2016-08-15 Emil Obreshkov <obreshko@cern.ch>
	* Small protection when reading lines from LCG_externals_<platform>.txt file
	* Tagging as AtlasLCG-00-00-65