package/gobject-introspection: export variables in g-ir-scanner

When building .gir .typelib files, the g-ir-scanner wrapper calls the host
g-ir-scanner. g-ir-scanner calls ccompiler.py, which searches for the following
environment variables:
    CPP, CC, CXX, CPPFLAGS, CFLAGS, CXXFLAGS, LDFLAGS

These environment variables are empty by default, and as such ccompiler.py
defaults to either using the system tools (CC, CXX, and CPP) or leaving
the variables blank (LDFLAGS, CFLAGS, and CPPFLAGS.)

For autotools packages, this issue does not occur because autotools uses
Makefile.introspection found on the staging directory in
usr/share/gobject-introspection-1.0/ which automatically exports the above
variables.

However, for meson, the above variables are not exported when meson calls
g-ir-scanner to build .gir and .typelib files, which results in linking errors.

Exporting these variables in the g-ir-scanner wrapper fixes these issues and
ensures all build systems can generate .gir and .typelib files properly.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>