BUG: build from source failed on Ubuntu 20.04
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Output of the build script

```bash
(pandas-dev) xxx@xxx-home-office:~/Workspace/pandas-VirosaLi$ python setup.py build_ext --inplace -j 4
running build_ext
building 'pandas._libs.interval' extension
gcc -pthread -B /home/xxx/miniconda3/envs/pandas-dev/compiler_compat -Wl,--sysroot=/ -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -DNPY_NO_DEPRECATED_API=0 -I./pandas/_libs/tslibs -Ipandas/_libs/src/klib -I/home/xxx/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/numpy/core/include -I/home/xxx/miniconda3/envs/pandas-dev/include/python3.8 -c pandas/_libs/interval.c -o build/temp.linux-x86_64-3.8/pandas/_libs/interval.o -Werror
building 'pandas._libs.tslibs.nattype' extension
gcc -pthread -B /home/xxx/miniconda3/envs/pandas-dev/compiler_compat -Wl,--sysroot=/ -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -DNPY_NO_DEPRECATED_API=0 -I./pandas/_libs/tslibs -I/home/xxx/miniconda3/envs/pandas-dev/lib/python3.8/site-packages/numpy/core/include -I/home/xxx/miniconda3/envs/pandas-dev/include/python3.8 -c pandas/_libs/tslibs/nattype.c -o build/temp.linux-x86_64-3.8/pandas/_libs/tslibs/nattype.o -Werror
pandas/_libs/tslibs/nattype.c:4998:18: error: ‘__pyx_pw_6pandas_5_libs_6tslibs_7nattype_4_NaT_11__div__’ defined but not used [-Werror=unused-function]
 4998 | static PyObject *__pyx_pw_6pandas_5_libs_6tslibs_7nattype_4_NaT_11__div__(PyObject *__pyx_v_self, PyObject *__pyx_v_other) {
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/interval.c:8301:18: error: ‘__pyx_pw_6pandas_5_libs_8interval_8Interval_25__div__’ defined but not used [-Werror=unused-function]
 8301 | static PyObject *__pyx_pw_6pandas_5_libs_8interval_8Interval_25__div__(PyObject *__pyx_v_self, PyObject *__pyx_v_y) {
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
cc1: all warnings being treated as errors
error: command 'gcc' failed with exit status 1

```

#### Problem description

Recently I decide to try out Ubuntu 20.04. The build process failed. See the output in the section above. I replaced my user name with xxx.

