This has worked in PHP 4 releases prior to 4.0.2:

After a PHP install, you could copy a PHP extension like gettext (which is properly prepared for being a self-contained extension) to a temporary directory and do
phpize
./configure --with-gettext=shared

This produced a working Makefile.
An example of another extension that would work like this: zlib.


Since 4.0.2, the phpize+./configure steps do not produce a Makefile.
If the extension's Makefile.in is copied to Makefile and the following lines

How PHP was installed:
Grabbed latest CVS, to ./buildconf and "./configure --with-apxs=/usr/sbin/apxs"; make install

On other words: A quite normal installation without any bells/whistles.


Analysis:
The only file which seems to be missing is the Makefile.
If Makefile.in is copied to Makefile and the following lines are added in the beginning of the file, then the self-contained extension compiles without errors:

top_srcdir   = .
top_builddir = $(top_srcdir)
srcdir       = $(top_srcdir)/
builddir     = $(top_srcdir)/
VPATH        = $(top_srcdir)/