Add configure option '--default-pic' to enable '-fPIC' by default

[umarcor]

Currently, GHDL can be built as:

OPT_FLAGS="-fPIC" \
LIB_CFLAGS="-fPIC" \
./dist/travis/build.sh -b llvm -p ghdl-llvm-fPIC

Then, sources must be analyzed with -fPIC in order to generate a PIE binary. E.g., in VUnit:

ui.set_compile_option("ghdl.flags", ['-fPIC'])
ui.set_sim_option("ghdl.elab_flags", ['-fPIC', '-Wl,-pie'])

---

This PR allows to simplify the commands above to:

CONFIG_OPTS="--enable-default-pie " \
./dist/travis/build.sh -b llvm -p ghdl-llvm-fPIC

and

ui.set_sim_option("ghdl.elab_flags", ['-Wl,-pie'])

I.e., if GHDL is built with --enable-default-pie, -fPIC is implicitly added (by default) to any ghdl -a or ghdl -e command.

This PR sets the default of variable default_pie in configure to false. It is enabled automatically if gcc was built with enable-default-pie, as it is now. The new feature is that it can be explicitly set through --enable-default-pie.

However, this alone is not enough to achieve the desired result. PIC_FLAGS must be added to GRT_FLAGS, in order to force all the objects in libghdl.a to be position independent.

Note that I changed the values of default_pie from no|yes to false|true, for coherency.

[umarcor]

@tgingold, I believe this is ready for reaview now. A test for this PR is available in #803. Running the script on fedora:29, ubuntu:trusty or ubuntu:xenial will not be able to load the binary as a shared library. Instead, if this branch is used (feat-default-pie), all of them will work.

Moreover, I think that #670 and #804 could be safely reverted with this PR, because the use cases are already covered with the now visible configure option; which is a much clearner solution, I think.

[tgingold]

I have a simple request: don't hijack enable-default-pie, just add a new option like enable-pic

[umarcor]

I have a simple request: don't hijack enable-default-pie, just add a new option like enable-pic

I updated it, but I don't know if that is what you mean.

EDIT

Maybe you meant to add Default_Pic to default_paths.ads.in?

[tgingold]

Thanks. I will adjust it a little bit so that libgrt is not built with -fpic by default.