Respect CMAKE_Fortran_FLAGS (fortran flags passed by the user).

[yurivict]

Systems should be able to set their preferred fortran flags.

[codecov]

Codecov Report

Merging #388 (a868621) into develop (1e7f52e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop    #388   +/-   ##
=======================================
  Coverage     62.1%   62.1%           
=======================================
  Files           87      87           
  Lines        25741   25741           
=======================================
  Hits         15997   15997           
  Misses        9744    9744           

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[jenkins-cscs]

Can one of the admins verify this patch?

[dev-zero]

The problem with the proposed solution is the ordering of flags: CMake always orders the build-type flags (release, debug, profiling) after the common flags:

cd /home/tiziano/work/cp2k/dbcsr/build/src && /usr/bin/gfortran -DNDEBUG -D__MPI_VERSION=3 -D__STATM_TOTAL -D__parallel -D__SHORT_FILE__=\"dbcsr_mpiwrap.F\" -I/home/tiziano/work/cp2k/dbcsr/src/base -I/home/tiziano/work/cp2k/dbcsr/src -I/home/tiziano/work/cp2k/dbcsr/build/src -I/usr/include -I/usr/lib/openmpi -ffree-form -std=f2008ts -fimplicit-none -Werror=aliasing -Werror=ampersand -Werror=c-binding-type -Werror=intrinsic-shadow -Werror=intrinsics-std -Werror=line-truncation -Werror=tabs -Werror=target-lifetime -Werror=underflow -Werror=unused-but-set-parameter -Werror=unused-but-set-variable -Werror=unused-variable -Werror=unused-dummy-argument -Werror=conversion -Werror=zerotrip -Werror=uninitialized -Wno-maybe-uninitialized -O3 -g -funroll-loops -pthread -fopenmp -c /home/tiziano/work/cp2k/dbcsr/build/src/mpi/dbcsr_mpiwrap.F -o CMakeFiles/dbcsr.dir/mpi/dbcsr_mpiwrap.F.o

... the -std=f2008ts comes from the common flags, -O3 comes from the release build-type. And with GCC later flags override earlier ones, so this patch would not allow to override the flags.

And this is the reason why distros like Gentoo build CMake packages with a custom build-type, which allows to override the flags already like this:

$ cmake -DCMAKE_BUILD_TYPE=custom -DCMAKE_Fortran_FLAGS_CUSTOM=-O2 ..
$ make VERBOSE=1
[...]
[ 49%] Building Fortran object src/CMakeFiles/dbcsr.dir/utils/dbcsr_min_heap.F.o
cd /home/tiziano/work/cp2k/dbcsr/build/src && /usr/bin/gfortran -D__MPI_VERSION=3 -D__STATM_TOTAL -D__parallel -D__SHORT_FILE__=\"dbcsr_min_heap.F\" -I/home/tiziano/work/cp2k/dbcsr/src/base -I/home/tiziano/work/cp2k/dbcsr/src -I/home/tiziano/work/cp2k/dbcsr/build/src -I/usr/include -I/usr/lib/openmpi -ffree-form -std=f2008ts -fimplicit-none -Werror=aliasing -Werror=ampersand -Werror=c-binding-type -Werror=intrinsic-shadow -Werror=intrinsics-std -Werror=line-truncation -Werror=tabs -Werror=target-lifetime -Werror=underflow -Werror=unused-but-set-parameter -Werror=unused-but-set-variable -Werror=unused-variable -Werror=unused-dummy-argument -Werror=conversion -Werror=zerotrip -Werror=uninitialized -Wno-maybe-uninitialized -O2 -pthread -fopenmp -c /home/tiziano/work/cp2k/dbcsr/build/src/utils/dbcsr_min_heap.F -o CMakeFiles/dbcsr.dir/utils/dbcsr_min_heap.F.o

I've added more documentation to docs/guide/2-user-guide/1-installation/2-cmake-build-recipes.md about this.