Do not compile fourth derivatives in LibXC (#1592)

We do not use fourth derivatives in our code.
Added a hint that "vanilla" LibXC is possible too.

INSTALL.md

@@ -197,6 +197,8 @@ the FFTW3 threading library libfftw3_threads (or libfftw3_omp) is required.
 ### 2k. libxc (optional, wider choice of xc functionals)
 
 - The version 5.1.0 (or later) of libxc can be downloaded from <https://www.tddft.org/programs/libxc>
+- CP2K does not make use of fourth derivates such that libxc may be configured
+  with './configure --disable-lxc \<other libxc configuration flags\>'.
 - During the installation, the directories `$(LIBXC_DIR)/lib`
   and `$(LIBXC_DIR)/include` are created.
 - Add `-D__LIBXC` to DFLAGS, `-I$(LIBXC_DIR)/include` to FCFLAGS

tools/toolchain/scripts/stage3/install_libxc.sh

@@ -43,7 +43,8 @@ case "$with_libxc" in
       [ -d libxc-${libxc_ver} ] && rm -rf libxc-${libxc_ver}
       tar -xzf libxc-${libxc_ver}.tar.gz
       cd libxc-${libxc_ver}
-      ./configure --prefix="${pkg_install_dir}" --libdir="${pkg_install_dir}/lib" > configure.log 2>&1
+      # CP2K does not make use of fourth derivatives, so skip their compilation with --disable-lxc
+      ./configure --prefix="${pkg_install_dir}" --libdir="${pkg_install_dir}/lib" --disable-lxc > configure.log 2>&1
       make -j $(get_nprocs) > make.log 2>&1
       make install > install.log 2>&1
       cd ..