[ET Trac] [Einstein Toolkit] #1735: support FFTW3_LIBS FFTW3_INC_DIRS FFTW3_LIB_DIRS in FFTW3 ExternalLibrary

[Einstein Toolkit]

#1735: support FFTW3_LIBS FFTW3_INC_DIRS FFTW3_LIB_DIRS in FFTW3 ExternalLibrary
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 Reporter:  rhaas                  |       Owner:                     
     Type:  defect                 |      Status:  new                
 Priority:  major                  |   Milestone:                     
Component:  EinsteinToolkit thorn  |     Version:  development version
 Keywords:  FFTW3                  |  
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 Right now FFTW3 claims to support FFTW3_LIBS (in configuration.ccl) but
 ignores the user provided value and always uses fftw3, it does not allow
 FFTW3_INC_DIRS or LLTW3_LIB_DIRS to be set at all.

 The attached patch changes this such that if those variables are specified
 in the option list, they override whatever detect.sh may want to use. This
 can be used to make use of the fftw implementation inside of MKL which
 stores its include files inside $MKL_ROOT/include/fftw and which should be
 used by sepcifying jsut {{{-mkl}} to the compiler rather than an actual
 library file.

 This is a bug since FFTW3_LIBS is documented but does not behave as
 documented. It is major since this should be fixed before the next
 release.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1735>
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