BuildSystem: Instead of $PJPROJECT_LIBS with s, use $PJPROJECT_LIB everywhere.

In the script ./configure,
xyz_LIB  is set by AST_PKG_CONFIG_CHECK and
xyz_LIBS is set by PKG_CHECK_MODULES within
AST_PKG_CONFIG_CHECK. Both are the same. In Asterisk normally the former and
only three times the latter was used. Let us use xyz_LIB without s, for
consistency with AST_EXT_LIB_CHECK. That eases understanding because now readers
do not have to know that xyz_LIB equals xyz_LIBS.

Change-Id: I7359860a5d730cdc784c2c48e501a082196434d3