Enable stand-alone build of public headers in target 'all' unless already specified otherwise

As discussed in rmatev/lb-stack-setup#83 (comment 7762532), this enables stand-alone builds of public headers in the target all.

Since this increases the build work a lot, developers can turn it off by passing -DGAUDI_TEST_PUBLIC_HEADERS_BUILD=FALSE when configuring the first time or directly editing the entry in build.<platform>/CMakeCache.txt.

/cc @jonrob