Autotools: Replace backticks with $(...) in php.m4

[petk]

This is a follow-up of GH-15639

[nikic]

This broke the apcu build with:

/usr/bin/ld: cannot find  -c: No such file or directory
collect2: error: ld returned 1 exit status

Old:

/bin/bash /home/nikic/php/php-src/libtool --silent --preserve-dup-deps --tag=CC --mode=compile gcc-12 -Iext/apcu/ -I/home/nikic/php/php-src/ext/apcu/ -I/home/nikic/php/php-src/main -I/home/nikic/php/php-src -I/home/nikic/php/php-src/ext/date/lib -I/usr/include/libxml2 -I/home/nikic/php/php-src/ext/mbstring/libmbfl -I/home/nikic/php/php-src/ext/mbstring/libmbfl/mbfl -I/home/nikic/php/php-src/TSRM -I/home/nikic/php/php-src/Zend  -D_GNU_SOURCE  -fno-common -Wstrict-prototypes -Wformat-truncation -Wlogical-op -Wduplicated-cond -Wno-clobbered -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -g -ffp-contract=off -fvisibility=hidden -O0 -Wimplicit-fallthrough=1 -DZEND_SIGNALS -Werror   -D_GNU_SOURCE -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1 -Wall -Wextra -Wno-unused-parameter -c /home/nikic/php/php-src/ext/apcu/apc.c -o ext/apcu/apc.lo  -MMD -MF ext/apcu/apc.dep -MT ext/apcu/apc.lo

New:

/bin/bash /home/nikic/php/php-src/libtool --silent --preserve-dup-deps --tag=CC --mode=compile gcc-12 -Iext/apcu/ -I/home/nikic/php/php-src/ext/apcu/ -I/home/nikic/php/php-src/main -I/home/nikic/php/php-src -I/home/nikic/php/php-src/ext/date/lib -I/usr/include/libxml2 -I/home/nikic/php/php-src/ext/mbstring/libmbfl -I/home/nikic/php/php-src/ext/mbstring/libmbfl/mbfl -I/home/nikic/php/php-src/TSRM -I/home/nikic/php/php-src/Zend  -D_GNU_SOURCE  -fno-common -Wstrict-prototypes -Wformat-truncation -Wlogical-op -Wduplicated-cond -Wno-clobbered -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -g -ffp-contract=off -fvisibility=hidden -O0 -Wimplicit-fallthrough=1 -DZEND_SIGNALS -Werror   \ -c /home/nikic/php/php-src/ext/apcu/apc.c -o ext/apcu/apc.lo  -MMD -MF ext/apcu/apc.dep -MT ext/apcu/apc.lo

Note the extra backslash before \.

[nikic]

It looks like a lot of the backtick expressions converted here contain \, which has notoriously different semantics between backticks and $().

[petk]

Ah, I see. The apcu is using a Makefile variable in the config.m4.

[petk]

I'll recheck this one. Yes, basically no simple way here but to go back to the backticks in php.m4 here:

  ifelse($5,,ac_extra=,[ac_extra=$(echo "$5"|$SED s#@ext_srcdir@#$ext_srcdir#g|$SED s#@ext_builddir@#$ext_builddir#g)])

From a quick PECL extensions check there is only APCU still having this \\$(APCU_CFLAGS) anomaly in there. For example, the GD extension's similar usage of CFLAGS was changed in PHP-8.4.