[cmake] Do not assume ninja

Summary: Currently, cmake always use ninja to build native executable. This patch changes this to use the same generator as the one provided by the user.

Test Plan:

cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win32.cmake
make
cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake
make

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1379