Use relative install paths for headers

[FtZPetruska]

Using relative paths in the calls to install allows the same build to be installed to several location without reconfiguring the project. This enables flexibility similarly to autotools':

./configure --prefix=/
make install DESTDIR=/foo
make install DESTDIR=/bar

Without this change:

cmake -S . -B build -D CMAKE_INSTALL_PREFIX=/foo -D CMAKE_TOOLCHAIN_FILE=...
cmake --build build
cmake --install build # All files installed in /foo
cmake --install build --prefix /bar # Library installed in /bar, headers installed in /foo

With this change:

cmake -S . -B build -D CMAKE_INSTALL_PREFIX=/foo -D CMAKE_TOOLCHAIN_FILE=...
cmake --build build
cmake --install build # All files installed in /foo
cmake --install build --prefix /bar # All files installed in /bar