Enable Building Both libjbpf.a and libjbpf.so with cmake option -DJBPF_STATIC=Both

[doctorlai-msrc]

This PR changes the cmake option JBPF_STATIC to 3 states: On, Off and Both.
When set to Both, both the libjbpf.a and libjbpf.so will be built

[xfoukas]

@doctorlai-msrc Can you please explain why we cannot build both the static and the dynamic library by default? Why

[doctorlai-msrc]

@doctorlai-msrc Can you please explain why we cannot build both the static and the dynamic library by default? Why

We can, however, building both would double the time.

[xfoukas]

Why would it double the time? Have you profiled this? Could you share some numbers?

[doctorlai-msrc]

Why would it double the time? Have you profiled this? Could you share some numbers?

Because building both will have two add_library. I did a few quick tests, however between tests, the time statistics do vary.

rm -rf * && rm -rf ../out/ && cmake ../ -DJBPF_STATIC=On
time bash -c " make -j"
real 1m16.577s
user 6m53.224s
sys 0m58.393s

rm -rf * && rm -rf ../out/ && cmake ../ -DJBPF_STATIC=Off
time bash -c " make -j"
real 1m46.668s
user 9m29.203s
sys 1m28.137s

rm -rf * && rm -rf ../out/ && cmake ../ -DJBPF_STATIC=Both
time bash -c " make -j"
real 1m58.854s
user 10m11.512s
sys 1m45.767s

[xfoukas]

The default (JBPF_STATIC=off) and the case when building both take similar time according to this profiling, so in my view we should build both by default, as it simplifies the build process.

[xfoukas]

I think this introduces an a bug for the static build. If this option is set to "BOTH", then jbpf will be built with -DJBPF_SHARED_LIB enabled, which would introduce a bug in the produced .a due to this preprocessor check:

jbpf/src/core/jbpf.c

Line 44 in f55747a

#ifdef JBPF_SHARED_LIB

When this option is set to "BOTH", we need to make sure that the static library is built without this flag enabled and the shared library is built with it.

[doctorlai-msrc]

Fixed.