meson CI: Add OmniOS

[neheb]

There's solaris code in here that's pretty much untested. Might as well do so.

CI is meson only currently. Added CMake support anyway. Cleaned up surrounding code.

[neheb]

  ../samples/geotag.cpp: In function 'int main(int, const char**)':
  ../samples/geotag.cpp:815:14: warning: unnecessary parentheses in declaration of 'program' [-Wparentheses]
    815 |     ::version(program);
        |              ^~~~~~~~~
  ../samples/geotag.cpp:815:14: note: remove parentheses
    815 |     ::version(program);
        |              ^~~~~~~~~
        |              -       -
  ../samples/geotag.cpp:815:15: warning: unused variable 'program' [-Wunused-variable]
    815 |     ::version(program);
        |               ^~~~~~~
  ../samples/geotag.cpp: At global scope:
  ../samples/geotag.cpp:630:5: warning: 'int {anonymous}::version(const char*)' defined but not used [-Wunused-function]
    630 | int version(const char* program) {
        |     ^~~~~~~

that's just weird.