Fix DTrace build with SystemTap and GCC defaults

We are experiencing an issue when building PHP with DTrace enabled with
SystemTap (see GH-11847).† SystemTap, unlike Oracle DTrace, Open DTrace
or their forks, relies on C compiler and preprocessor. We have recently
aligned both of these with compiler and preprocessor we use to build
regular C source code (see GH-11643).‡ We set CPP environment variable
to the value of our CPP macro which defaults to C preprocessor detected
by configure script. Similarly, we set CC environment variable to the
value of our CC macro which defaults to C compiler detected by configure
script. C compiler flags have already been in place since versions
5.4.20 and 5.5.4 from 2013-09-18.

We had modified all dtrace invocation in the same way to make it look
consistent. However, SystemTap dtrace needs C preprocessor only when it
generates header files (-h option) with preprocessing (-C option).
Similarly, it needs C compiler only when it generates object files with
probe definitions (-G option). Therefore, we have recently stopped
setting all these environment variables when invoking dtrace. We set
only what is necessary for particular action. We hope this makes this
fix simpler and clearer without introducing additional complexity to our
build system.

The cause of our issue is the fact that GCC preprocessor, when invoked
via “gcc” binary, detects the language for preprocessing from source
file extensions. Because DTrace probes and providers for
statically-defined tracing are in files with “.d” extension, GCC
detection fails because it probably thinks it is processing proper D
programing language source code (somewhat different from what DTrace
uses) which has no C-style preprocessing. People have noticed, because
configure script on systems with GCC sets CPP make macro to “gcc -E”.
Therefore practically everyone, who does not override C preprocessor,
who is building PHP with DTrace enabled on Linux with SystemTap,
experiences this issue.

We force C language when passing C preprocessor as set by the user or
detected by the configure script. SystemTap dtrace expects C language
preprocessor because pragmas in D programming language used by DTrace
are the same #pragma directives known from C and similar languages.

We use -x option from GCC to force C. Other compilers suites and their
preprocessors (most notably Clang) recognize this option too. Many
probably do not. However, this issue is specific to SystemTap, which is
itself specific to Linux and has to be build using the same compiler as
Linux kernel, which only supports building using GCC-compatible
compilers. We can definitely imagine someone building PHP with DTrace
enabled using SystemTap and wanting to use compiler suite or
preprocessor which is different from what they used to build SystemTap
itself or their Linux kernel and which does not recognize -x option.
However, we do not think it is reasonable to accommodate for such use
case. Properly solving this would require implementing non-trivial m4
macros, which is probably not warranted given this affects 2 lines of
make commands…

This has no effect on Oracle DTrace, Open DTrace or any of their forks
because they do not allow customization of preprocessor nor compiler.
They simply ignore CPP, CC, and CFLAGS environment variables, so it does
not matter what values we set there.

Fixes GH-11847

† #11847
‡ #11643

Author: f4z4on

build/php.m4

@@ -2389,7 +2389,7 @@ dnl overwritten (Bug 61268).
 $abs_srcdir/$ac_provsrc:;
 
 $ac_bdir[$]ac_hdrobj: $abs_srcdir/$ac_provsrc
-	CPP="\$(CPP)" dtrace -h -C -s $ac_srcdir[$]ac_provsrc -o \$[]@.bak && \$(SED) -e 's,PHP_,DTRACE_,g' \$[]@.bak > \$[]@
+	CPP="\$(CPP) -xc" dtrace -h -C -s $ac_srcdir[$]ac_provsrc -o \$[]@.bak && \$(SED) -e 's,PHP_,DTRACE_,g' \$[]@.bak > \$[]@
 
 \$(PHP_DTRACE_OBJS): $ac_bdir[$]ac_hdrobj
 

ext/oci8/config.m4

@@ -124,7 +124,7 @@ dnl overwritten (Bug 61268).
 PHP_EXT_SRCDIR([oci8])/$ac_provsrc:;
 
 $ac_bdir[$]ac_hdrobj: $ac_srcdir[$]ac_provsrc
-	CPP="\$(CPP)" dtrace -h -C -s $ac_srcdir[$]ac_provsrc -o \$[]@.bak && \$(SED) -e 's,PHPOCI_,DTRACE_,g' \$[]@.bak > \$[]@
+	CPP="\$(CPP) -xc" dtrace -h -C -s $ac_srcdir[$]ac_provsrc -o \$[]@.bak && \$(SED) -e 's,PHPOCI_,DTRACE_,g' \$[]@.bak > \$[]@
 
 \$(OCI8_DTRACE_OBJS): $ac_bdir[$]ac_hdrobj