FTBFS: undefined reference to pthread

Excerpt from buildlog:

make[5]: Entering directory `/build/buildd/afnix-2.2.0/src/clt/axi/exe'
g++ -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall -fno-builtin -MMD -pthread -fPIC -nostdinc -nostdinc++ -march=i486 -I. -I../../../../bld/hdr/eng -I../../../../bld/hdr/std -I../../../../bld/hdr/plt -o Main.o -c Main.cpp
g++ -Wl,-rpath=/usr/lib/afnix,-rpath-link,/usr/lib/afnix -Wl,-Bsymbolic-functions -Wl,-z,relro -pthread -o axi Main.o -L../../../../bld/lib -lafnix-eng -lafnix-std -lafnix-plt -ldl -lm -lcurses
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_mutex_trylock'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_mutexattr_settype'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_key_create'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_once'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_getspecific'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_mutexattr_destroy'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_create'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_mutexattr_init'
../../../../bld/lib/libafnix-plt.so: undefined reference to `pthread_setspecific'
collect2: error: ld returned 1 exit status
make[5]: *** [axi] Error 1
make[5]: Leaving directory `/build/buildd/afnix-2.2.0/src/clt/axi/exe'
make[4]: *** [build] Error 2
make[4]: Leaving directory `/build/buildd/afnix-2.2.0/src/clt/axi'
make[3]: *** [build] Error 2
make[3]: Leaving directory `/build/buildd/afnix-2.2.0/src/clt'
make[2]: *** [build] Error 2
make[2]: Leaving directory `/build/buildd/afnix-2.2.0/src'
make[1]: *** [world] Error 2
make[1]: Leaving directory `/build/buildd/afnix-2.2.0'
make: *** [build-stamp] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2
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Build finished at 20130922-2037
FAILED [dpkg-buildpackage died]
******************************************************************************
Finished at 20130922-2037