giac: fix #include <giac/giac.h> in musl

[tornaria]

As reported in #45708 (comment), including <giac/giac.h> fails in musl because the HAVE_TGAMMAF macro from autotools is not available at this time.

Here we include a patch adapted from geogebra/giac@618a5de which is the solution upstream will include in next release of giac.

We need to fix this now so we can build sagemath in musl!

Testing the changes

• I tested the changes in this PR: YES

[tornaria]

Note all the added patch does is moving the definition of inline float fgamma() verbatim from first.h to usual.cc which is the only place where it is used.

In the template I added a simple test in post_install() that compiles #include <giac/giac.h> to make sure it works.

[tornaria]

My bad, I was using gcc to test compilation using #include <giac/giac.h> but that doesn't work in cross. Fixing it now by using ${CC} instead.