[petsc-users] configure error with fftw and without mpi

[Fabio Rossi]

I am trying to compile petsc with the following options (Gentoo build system):

./configure --prefix=/usr --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --mandir=/usr/share/man --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc --localstatedir=/var/lib --libdir=/usr/lib64/petsc/lib64 scrollOutput=1 FFLAGS=-O2 -pipe -fPIC CFLAGS=-march=core2 -O2 -pipe -fPIC CXXFLAGS=-march=core2 -O2 -pipe -fPIC LDFLAGS=-Wl,-O1 -Wl,--as-needed --prefix=/usr/lib64/petsc --with-shared-libraries --with-single-library --with-clanguage=c --with-petsc-arch=linux-gnu-c-opt --with-precision=double --with-gnu-compilers --with-blas-lapack-lib=-lblas -llapack  --with-debugging=0 --with-mpi=0 --with-cc=x86_64-pc-linux-gnu-gcc --with-cxx=x86_64-pc-linux-gnu-g++ --with-fortran=1 --with-fc=x86_64-pc-linux-gnu-gfortran --with-mpi-compilers=0 --with-scalar-type=real --with-windows-graphics=0 --with-matlab=0 --with-cmake:BOOL=1 --with-pthread=1 --with-afterimage=0 --without-hdf5 --with-hypre=0 --without-suitesparse --with-superlu=0 --with-x --with-x11 --with-ptscotch=0
  --with-
 scalapack=0 --without-mumps --with-imagemagick=0 --with-python=0 --with-boost --with-fftw

so MPI support is disabled, and I get the following configure error:

TESTING: consistencyChecks from config.packages.fftw(/var/tmp/portage/sci-mathematics/petsc-3.15.0/work/petsc-3.15.0/config/BuildSystem/config/package.py:963)
TESTING: checkDependencies from config.packages.fftw(/var/tmp/portage/sci-mathematics/petsc-3.15.0/work/petsc-3.15.0/config/BuildSystem/config/package.py:872)
*******************************************************************************
         UNABLE to CONFIGURE with GIVEN OPTIONS    (see configure.log for details):
-------------------------------------------------------------------------------
Did not find package MPI needed by fftw.
Enable the package using --with-mpi

Isn't MPI optional? Where is the error coming from?

If I disable fftw then it compiles successfully.

Fabio