Minor problem in the m4/acx_pthread.m4

[ningli-amd]

This is related to #17

In that issue, the link error was caused by missing -lpthread in the command line. The configure script actually failed to detecting the correct library flag. This is part of the log:

checking pthread.h usability... yes
checking pthread.h presence... yes
checking for pthread.h... yes
checking for the pthreads library -lpthreads... no
checking whether pthreads work without any flags... no
checking whether pthreads work with -Kthread... no
checking whether pthreads work with -kthread... no
checking for the pthreads library -llthread... no
checking for the pthreads library -lpthread... no
checking whether pthreads work with -pthread... no
checking whether pthreads work with -pthreads... no
checking whether pthreads work with -mthreads... no
checking whether pthreads work with --thread-safe... no
checking whether pthreads work with -mt... no

Note that checks for all possible pthread flags failed. Same detection with the old macro before 1887a62 was successful.

Looking into m4/acx_pthread.m4, the use of AC_LANG_CALL macro at line 173, 202 and 206 should be replaced by AC_LANG_PROGRAM.