When building with SQLITECPP_INTERNAL_SQLITE=ON the SQLite amalgamation
source is used for generating the library. Using one big source file
means all the library code will be put in a single section. When
building statically linked executables the entire section will be
linked even if a small portion of the library is actually used.
This commit addresses this issue by setting the -ffunction-sections
compiler option when building the library. As each function is placed in
a section of its own the linker, when passed the --gc-sections, will
throw away unused sections (functions) and reduce the executable size.
