configure: Use enable_weak when enabling pic

This allows it to be overridden, either by the user on the command
line, or by other sections of the configure script.

Signed-off-by: Martin Storsjö <martin@martin.st>
configure: Explicitly disable PIC when targeting win32/mingw

These platforms do not have any notion of PIC. On some compilers,
enabling pic produces a number of warnings.

This avoids trying to produce PIC loads in the ARM assembly - there
are no relocation types in PE/COFF that correspond to
BFD_RELOC_32_PCREL (R_ARM_REL32 in ELF).

As a side-effect, this avoids enabling PIC on mingw64, getting rid
of the warnings about PIC not having any effect on that platform.

Signed-off-by: Martin Storsjö <martin@martin.st>