BuildSystem: For consistency, avoid double-checking via if clauses.

In the script ./configure, AST_EXT_LIB_CHECK and AST_PKG_CONFIG_CHECK first test
whether parameter 1 was already found. Consequently, an if-test on PBX_ just a
line below is redundant, if exactly the same parameter 1 is used again.

No performance gain is expected by this change. However, because this strategy
is used all over in ./configure except for two places, this change aims to
create more consistency: Only do something different if there is a reason to do
so.

Change-Id: I4a6f48127b7af3a48168c917e888be1f70625027