[SECP256K1] CMake: set default build configuration and optimization

Summary:
The optimization flags when configured with autotools are as set like:

• Always use -g, unless something is set in the CFLAGS as an

environment variable.

• Append -O3 if coverage is not enabled, -O0 otherwise.

This diff adds some kind of similar behavior to CMake:

• If no build configuration is defined, use RelWithDebInfo (release

with debug information).

• Set the flags to -g -O3 for this configuration.

With this diff, the CMake default build will have the same flags than
autotools by default, i.e. -g -O3. It is still possible to select a
different build configuration (and then use the associated flags), and
also pass additional flags through the CMAKE_C_FLAGS cache variable.

Note: It is not possible to select the optimization level with
autotools; since the -O3 flag is appended to the CFLAGS it will take
precedence over any passed value. As a consequence, I don't know what
the CFLAGS=-O0 case is testing in the Travis build, but it is not
testing against a non optimized binary.

Test Plan:

cmake -GNinja ..
ninja -v secp256k1

Check the -g -O3 flags are used.

ninja check-secp256k1

Repeat the same steps when building secp256k1 as a standalone project,
expect the same result.

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja -v secp256k1

Check the -g3 -O2 flags are used.

ninja check-secp256k1

Repeat the same steps when building secp256k1 as a standalone project,
expect the flag -g only (CMake defaults for the Debug configuration).

Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/643874641).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5106