configure: fix RPM_RELEASE

Use git to get RPM_RELEASE only if this is a git repo
clone and if the git command is available on the system.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
configure: remove -m4_include(m4/acx_pthread.m4)

Since we use already AC_CONFIG_MACRO_DIR, no need to include m4/acx_pthread.m4
extra.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>