OK I wrongly assumed that the 3 bash packages, being listed in the system packages, did not need to be explicitly installed in order to be used by (gnu build system). So now i've corrected that and they're in my user path.
arnaud@brix ~$ guix package --search-paths
export PATH="/home/arnaud/.guix-profile/bin:/home/arnaud/.guix-profile/sbin"
export BASH_LOADABLES_PATH="/home/arnaud/.guix-profile/lib/bash"
export GUIX_GTK2_PATH="/home/arnaud/.guix-profile/lib/gtk-2.0"
export XDG_DATA_DIRS="/home/arnaud/.guix-profile/share"
export GIO_EXTRA_MODULES="/home/arnaud/.guix-profile/lib/gio/modules"
export PKG_CONFIG_PATH="/home/arnaud/.guix-profile/lib/pkgconfig:/home/arnaud/.guix-profile/share/pkgconfig"                                                                                                  
export C_INCLUDE_PATH="/home/arnaud/.guix-profile/include"
export CPLUS_INCLUDE_PATH="/home/arnaud/.guix-profile/include"
export LIBRARY_PATH="/home/arnaud/.guix-profile/lib"
export INFOPATH="/home/arnaud/.guix-profile/share/info"
arnaud@brix ~$

Still, the result is the same.
As Thomas kindly noticed, there actually is no configure script provided anywhere in the source code, so perhaps the Makefile can be directly invoked, skipping the usual configure stage ?
In the meantime I'm going to try the environment definition route as well.
Thanks for your help, really appreciated !

2018-04-03 21:41 GMT+02:00 Thomas Sigurdsen <thomas.sigurdsen@gmail.com>:
Haha, I see that I added bash because I misread the error and someone
mentioned bash was needed.

Bash is not needed. You can delete the input for bash both the use-module and
the native-input bit. Be careful of matching parenthesis.

On Tue, 3 Apr 2018 21:27:16 +0200
Thomas Sigurdsen <thomas.sigurdsen@gmail.com> wrote:

> Hi Arnaud.
>
> Had a look at your package, it does not have the configure script where
> build-system gnu is expecting. I couldn't see it anywhere else either.
>
> I've made a few changes that I attached. With this you can put the file in a
> directory that you append to your $GUIX_PACKAGE_PATH and then just run 'guix
> build freefilesync'.
>
> I see the License file has three other licenses in addition to GPL3. But the
> names look like free software too (though I'm not a lawyer). No idea how we
> handle cases like that.
>
> Also, the rest of the code tree looks very non-standard as far as the tools
> I'm used go... Which is probably why you are having trouble building it.
>
> Hope that helps a little.
>
> Thomas
>
> On Tue, 3 Apr 2018 19:14:34 +0200
> Arnaud B <arnaud.beaudhuin@gmail.com> wrote:
>
> > Hello everyone,
> >
> > Well I'm happy to report that thanks to your advice I made some progress.
> > Using the scm file attached and the command :
> > *guix build -L /home/arnaud/guix_packages/gnu/packages/ -K -f
> > freefilesync.scm*
> > the build process starts as it should, the zip file is downloaded, checked
> > and unziped as needed, and the cpp files compile normally.
> > It stops at the configure script stage, apparently not finding the
> > configure command.
> > H
> > ​ere is the relevant part :
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > *phase `unpack' succeeded after 0.3 secondsstarting phase
> > `patch-usr-bin-file'phase `patch-usr-bin-file' succeeded after 0.0
> > secondsstarting phase `patch-source-shebangs'phase `patch-source-shebangs'
> > succeeded after 0.2 secondsstarting phase `configure'source directory:
> > "/tmp/guix-build-freefilesync-9.9.drv-0/source" (relative from build:
> > ".")build directory:
> > "/tmp/guix-build-freefilesync-9.9.drv-0/source"configure flags:
> > ("CONFIG_SHELL=/gnu/store/icz3hd36aqpjz5slyp4hhr8wsfbgiml1-bash-minimal-4.4.12/bin/bash"
> > "SHELL=/gnu/store/icz3hd36aqpjz5slyp4hhr8wsfbgiml1-bash-minimal-4.4.12/bin/bash"
> > "--prefix=/gnu/store/v07x6jfph9yy7bf54ibdb9z08snjn6lh-freefilesync-9.9"
> > "--enable-fast-install"
> > "--build=x86_64-unknown-linux-gnu")
> > /gnu/store/icz3hd36aqpjz5slyp4hhr8wsfbgiml1-bash-minimal-4.4.12/bin/bash:
> > ./configure: No such file or
> > directory                                              phase `configure'
> > failed after 0.0 secondsnote: keeping build directory
> > `/tmp/guix-build-freefilesync-9.9.drv-1'builder for
> > `/gnu/store/r13m30fdxvxindynh4161ca498lj3y09-freefilesync-9.9.drv' failed
> > with exit code 1@ build-failed
> > /gnu/store/r13m30fdxvxindynh4161ca498lj3y09-freefilesync-9.9.drv - 1
> > builder for
> > `/gnu/store/r13m30fdxvxindynh4161ca498lj3y09-freefilesync-9.9.drv' failed
> > with exit code 1                     guix build: error: build failed:
> > build of
> > `/gnu/store/r13m30fdxvxindynh4161ca498lj3y09-freefilesync-9.9.drv' failed*
> > This is surprising to me as, according to the manual
> > <https://www.gnu.org/software/guix/manual/html_node/Defining-Packages.html>,
> > configure is part of the usual set of tools found in the gnu build system.
> > Let met quote :
> > *"The (...) gnu-build-system represents the familiar GNU Build System,
> > where packages may be configured, built, and installed with the usual
> > ./configure && make && make check && make install command sequence."*
> > I checked that bash, bash-minimal, bash-completion are all found in the
> > store (actually in 3 differents versions after guix puil I guess).
> >
> > So again I rely on your knowledge, hoping to get through eventually.
> >
> > Many thanks in advance
> >
> > Arnaud
> >
> > PS : I reworded the package name and description as per the guidelines.
> >
> >
> > 2018-04-02 21:53 GMT+02:00 Arnaud B <arnaud.beaudhuin@gmail.com>:
> >
> > > ​Hello Pierre and Oleg
> > >
> > > Thanks a lot for the detailed and helpful answers, I will do accordingly
> > > tomorrow night after work and report.
> > > Cheers
> > >
> > >
>