Update autoconf macros from autoconf-archive 2012.09.08

cfea4ee ax_append_flag: Split file into one macro per file
5f9f40b Use one implementation for C/C++ in ax_cflags_*.m4
48a6e62 ax_cflags_*: Use AX_APPEND_FLAG macro
dc74e8f Updated serial numbers, and re-formatted the macro to be consistent with the rest of the archive.
7d5e931 AC_REQUIRE AX_APPEND_FLAG when appropriate.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Update autoconf macros from pkg-config 0.28

5fc77a9 Add autoconf macros to help packagers install .pc files
5b463c9 Include PKG_CHECK_VAR macro for reading variables in .pc files

Signed-off-by: Anders Kaseorg <andersk@mit.edu>