This happens with an image produced by lede-imagebuilder-x86-64.Linux-x86_64 that was download August 2nd:
pip install argon2-cffi produces this error:
Package libffi was not found in the pkg-config search path.
Perhaps you should add the directory containing `libffi.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libffi' found
unable to execute 'ccache_cc': No such file or directory
unable to execute 'ccache_cc': No such file or directory
No working compiler found, or bogus compiler options
passed to the compiler from Python's distutils module.
See the error messages above.
(If they are about -mno-fused-madd and you are on OS/X 10.8,
see http://stackoverflow.com/questions/22313407/ .)
Cleaning up...
Removing source in /tmp/pip-build-QZQi0j/argon2-cffi
gcc is installed though.
More debug info:
unable to execute ['ccache_cc', '-fno-strict-aliasing', '-Os', '-pipe', '-fno-caller-saves', '-fno-plt', '-fhonour-copts', '-Wno-error=unused-but-set-variable', '-Wno-error=unused-result', '-iremap/build/lede-snapshots/x86_64/build/sdk/build_dir/target-x86_64_musl/Python-2.7.13:Python-2.7.13', '-Wformat', '-Werror=format-security', '-fstack-protector', '-D_FORTIFY_SOURCE=1', '-Wl,-z,now', '-Wl,-z,relro', '-DNDEBUG', '-fno-inline', '-DNDEBUG', '-Os', '-pipe', '-fno-caller-saves', '-fno-plt', '-fhonour-copts', '-Wno-error=unused-but-set-variable', '-Wno-error=unused-result', '-iremap/build/lede-snapshots/x86_64/build/sdk/build_dir/target-x86_64_musl/Python-2.7.13:Python-2.7.13', '-Wformat', '-Werror=format-security', '-fstack-protector', '-D_FORTIFY_SOURCE=1', '-Wl,-z,now', '-Wl,-z,relro', '-fPIC', '-c', '_configtest.c', '-o', '_configtest.o']: No such file or directory