We are experiencing an issue when building PHP with DTrace enabled with
SystemTap (see phpGH-11847).† The issue is caused by inappropriate use C
preprocessor detected by GNU Autoconf in our “configure” script. C
preprocessor configuration found by AC_PROG_CPP macro is portable only
to run on files with “.c” extension.‡ However, statically-defined tracing
is described by D programs with “.d” extension which causes the issue.
We experience this even on typical Linux distribution with GNU Compiler
Collection (GCC) unless we override the defaults detected by our
“configure” script.

Many major Linux distributions use SystemTap to provide “dtrace”
utility. It relies on both external C preprocessor and external C
compiler. C preprocessor can be customized via CPP environment variable.
Similarly, C compiler can be customized via CC environment variable. It
also allows customization of C compiler flags via CFLAGS environment
variable. We have recently aligned both CPP and CC environment variable
with C preprocessor and C compiler we use to build regular C source code
as provided by our “configure” script (see phpGH-11643).* We wanted to
allow cross-compilation on Linux for which this was the only blocker. C
compiler flags from CFLAGS_CLEAN macro have already been in place since
versions 5.4.20 and 5.5.4 from 2013-09-18.

We had modified all “dtrace” invocations in the same way to make it look
consistent. However, only the C compiler (CC environment variable) is
necessary to for cross-compilation. There have never been any reported
issue with the C preprocessor. We acknowledge it would be great to allow
C preprocessor customization as well. However, the implementation would
require a lot of effort to do correctly given the limitations of
AC_PROG_CPP macro from GNU Autoconf. This would be further complicated
by the fact that all DTrace implementations, not just SystemTap, allow C
preprocessor customization but Oracle DTrace, Open DTrace, and their
forks do it differently. Nevertheless, they all default to “cpp” utility
and they all have or had been working fine. Therefore, we believe simply
removing CPP stabilizes “dtrace” invocation on Linux systems with
SystemTap and aligns it with other system configurations on other
platforms, until someone comes with complete solution with custom “m4”
and “make” macros, while our build system on Linux with SystemTap
supports cross-compilation.

Fixes phpGH-11847

† php#11847
‡ https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.71/autoconf.html#index-AC_005fPROG_005fCPP-1
* php#11643