Modernize libcudf basic example CMakeFile; updates CI build tests

[isVoid]

Follow up of #7671

This PR addresses review comments from #7671 (comment), modernizing the CMakeFile in libcudf basic example.

This PR also updates build tests for examples to make sure they are tested after both regular/project flash code path.

[codecov]

Codecov Report

Merging #8568 (9fad262) into branch-21.08 (1de662f) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 9fad262 differs from pull request most recent head 34958d7. Consider uploading reports for the commit 34958d7 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.08    #8568      +/-   ##
================================================
+ Coverage         82.57%   82.62%   +0.04%     
================================================
  Files               109      109              
  Lines             17817    17859      +42     
================================================
+ Hits              14713    14756      +43     
+ Misses             3104     3103       -1     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/lists.py	86.53% <100.00%> (+0.26%)	⬆️
python/cudf/cudf/core/dtypes.py	93.98% <100.00%> (+1.03%)	⬆️
python/cudf/cudf/utils/dtypes.py	82.87% <0.00%> (+0.38%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1de662f...34958d7. Read the comment docs.

[harrism]

@gpucibot merge