Build with default options is broken on Ubuntu 18.04

[traversaro]

In particular the problem is

In file included from /home/runner/work/libxml2-cmake-buildsystem/libxml2-cmake-buildsystem/build/_deps/libxml2upstream-src/xpath.c:469:0:
/home/runner/work/libxml2-cmake-buildsystem/libxml2-cmake-buildsystem/build/_deps/libxml2upstream-src/timsort.h:74:33: warning: type defaults to ‘int’ in declaration of ‘uint64_t’ [-Wimplicit-int]
 static int compute_minrun(const uint64_t);
                                 ^~~~~~~~
/home/runner/work/libxml2-cmake-buildsystem/libxml2-cmake-buildsystem/build/_deps/libxml2upstream-src/timsort.h:129:42: error: unknown type name ‘uint64_t’
 static __inline int compute_minrun(const uint64_t size) {
                                          ^~~~~~~~
/home/runner/work/libxml2-cmake-buildsystem/libxml2-cmake-buildsystem/build/_deps/libxml2upstream-src/timsort.h: In function ‘compute_minrun’:
/home/runner/work/libxml2-cmake-buildsystem/libxml2-cmake-buildsystem/build/_deps/libxml2upstream-src/timsort.h:133:9: error: unknown type name ‘uint64_t’
   const uint64_t mask = (1ULL << shift) - 1;
         ^~~~~~~~
CMakeFiles/xml2.dir/build.make:283: recipe for target 'CMakeFiles/xml2.dir/_deps/libxml2upstream-src/xpath.c.o' failed
make[2]: *** [CMakeFiles/xml2.dir/_deps/libxml2upstream-src/xpath.c.o] Error 1
make[1]: *** [CMakeFiles/xml2.dir/all] Error 2
CMakeFiles/Makefile2:72: recipe for target 'CMakeFiles/xml2.dir/all' failed
make: *** [all] Error 2
Makefile:129: recipe for target 'all' failed
##[error]Process completed with exit code 2.

See https://github.com/robotology/libxml2-cmake-buildsystem/pull/1/checks?check_run_id=205628528 .