trousers: fix build with musl

The trousers code uses getpwent_r, which is not available in musl.
Detect the availability of getpwent_r in the trousers build system, and
use it conditionally.

This broke the build of tpm-tools because linking with libtspi.so
failed.

Fixes:

  http://autobuild.buildroot.net/results/830fc20c68a0653afa5567edffc2ededc4e45cc6



Runtime-tested by running tpm_version in a chroot and verifying that it
creates a "user.data" file with both the Buildroot-provided CodeSourcery
and Musl toolchains on x86_64.

Signed-off-by: Noé Rubinstein <nrubinstein@aldebaran.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>