don't explicitly --enable-pic for ZTS

[henderkes]

When libtool sees --enable-pic, it goes through flags, adds -fpic if not supplied and removes -fpie from cflags, putting it in pie_flags instead, which aren't passed to the compilation. This results in __PIC__ being defined but __PIE__ undefined, causing initial-exec to be selected instead of local-exec. Every tls variable access requires 2 instructions instead of one.

# Initial-exec: load offset from GOT, then load TLS value
movq tls_cache@gottpoff(%rip), %rax
movq %fs:(%rax), %rax

# Local-exec: offset encoded directly in the instruction
movq %fs:tls_cache@tpoff, %rax

When not passing --enable-pic, our cflags which contain -fpic fpie anyways are honoured and php selects local-exec.

[tianon]

You put some good detail in this PR description so I think we should link to it, and I think we can go a little bit tighter on the note:

Suggested change

	{{ ) else ( -}}
	# Forcing PIC also removes -fpie from executable objects, preventing local-exec TLS.
	# So do not --enable-pic
	{{ ) end -}}
	{{ ) else ( -}}
	# no --enable-pic; forcing PIC also removes -fpie from executable objects, preventing local-exec TLS: https://github.com/docker-library/php/pull/1692
	{{ ) end -}}

[henderkes]

I've applied the suggestion and updated the PR comment.

I will also upstream a fix to libtool, but it won't land in php 8.6.