-DCMAKE_BUILD_TYPE=xxx sets some C compiler flags according to the build type.
-DBUILD_CONFIG was completely overwriting them in some compiler / arch
combinations and not in others. Make it consistently "append-only", not
overwrite.
Also, enforce the same set of flags for Release and RelWithDebInfo.
This reverts ff1f611a0d as it is no longer
necessary.
Avoid assert()
By default, CMAKE_BUILD_TYPE RelWithDebInfo or Release implies
-DNDEBUG, which disables the assert() macro. MariaDB is deviating
from that. Let us be explicit to use assert() only in debug builds.
