--libdir=DIR does not work properly

[Fellypao]

Dear maintainers,

The option --libdir=DIR seems to not work properly when passed to the command ./configure , since some files still being installed in /usr/lib (I used --prefix=/usr when building)

from the Makefile (after running ./configure ):
.......
ff_prefix_dir = /usr/lib/ff++/4.12
ff_prefix_dir_etc = /usr/lib/ff++/4.12/etc
ff_prefix_dir_example = ${datarootdir}/FreeFEM/4.12
ff_prefix_dir_include = /usr/lib/ff++/4.12/include
ff_prefix_dir_lib = /usr/lib/ff++/4.12/lib
ff_prefix_dir_lib_mpi = /usr/lib/ff++/4.12/lib/mpi
ff_unzip = yes
ff_wget = yes
host = x86_64-pc-linux-gnu
host_alias =
host_cpu = x86_64
host_os = linux-gnu
host_vendor = pc
htmldir = ${docdir}
includedir = ${prefix}/include
infodir = ${datarootdir}/info
install_sh = ${SHELL} /tmp/SBo/FreeFem-sources-4.12/install-sh
libdir = /usr/lib64
.......

As it can be seen, libdir was set as /usr/lib64 . However, some of the options ff_prefix_dir* remain starting with /usr/lib and, consequently, the corresponding files are installed in /usr/lib. But I expected to find them in /usr/lib64
I guess the option libdir should be set as a variable in Makefile before the options ff_prefix_dir* in a way that the last ones would be set as:
ff_prefix_dir* = ${libdir}/*