Fixes build for windows with MSVC compiler

[vicalejuri]

Hi Andrew! First, many thanks for your work , it works great! 👏

This pull request fixes compilation errors in the unit tests for windows 10 and Visual Studio 14 2015.

To use the math library in MSVC, the _USE_MATH_DEFINES should be defined.
And also, there was an error in linking the math library, which is actually unnecessary for MSVC.

Additionally, i've included a compilation target for the example

Please let me know if you have problems or if this patch needs more work/refactor.

[codecov-io]

Codecov Report

Merging #14 into master will decrease coverage by 2.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   98.95%   96.44%   -2.52%     
==========================================
  Files           3        4       +1     
  Lines         384      394      +10     
==========================================
  Hits          380      380              
- Misses          4       14      +10

Impacted Files	Coverage Δ
tests/stableapi_tests.cpp	100% <ø> (ø)	⬆️
examples/demo.c	0% <0%> (ø)

---

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 2406a22...b320469. Read the comment docs.