* configure.ac: Reorder some code.  Improve pthread test to take libc
in account.  Generate NPTH_CONFIG_LIBS from tests.
* src/npth-config.in: Use NPTH_CONFIG_LIBS.
* w32/npth-config.in: Ditto.
* tests/Makefile.am (AM_CPPFLAGS, AM_LDFLAGS): Don not pass -pthread.