__VA_OPT__ is not well supported by compilers other than gcc (see clang), most
likely as it is not yet standardised, and appears much newer than ,##. Even
though ,## is not standardised either, it is more likely to be supported by a
given compiler.

With GCC 10 several things got changed. One of the things is the
-fno-common default which prevented the code to link cause of multiple
definition issues.
Declared the variables extern and defined them in c-file to avoid the linker error.
Details can be found here: https://gcc.gnu.org/gcc-10/porting_to.html

It appears that Arch Linux has either moved harfbuzz headers, or is newly
depending on harbuzz from pango. This patch includes the harfbuzz include dir
in CFLAGS.