internal GPTL with autotools build

[rjdave]

When I add --enable-timing in the autotools build configure line for PIO 2.5.4 on a system that does not have an existing GPTL library built, configure fails:

checking gptl.h usability... no
checking gptl.h presence... no
checking for gptl.h... no
checking for GPTLinitialize in -lgptl... no
configure: error: Can't find or link to the GPTL library.

instead of falling back to building the gptl library provided in the PIO source code. The cmake build does not suffer from this shortcoming and happily builds the GPTL shipped with PIO.

[edwardhartnett]

Correct. That is a feature not a bug.

Shipping code from the GPTL project is incorrect. Get your GPTL from GPTL and get PIO from PIO.

Provide the location to GPTL as you would with any other library, in CPPFLAGS and LDFLAGS.