don't explicitly --enable-pic for ZTS

[henderkes]

for some reason libtool strips -fpie from cflags. this results in initial-exec being selected and every tls variable access requiring 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