fix: stop version.h dropping zero-valued version components

[skilledwolf]

version.h.in declared the version components with cmakedefine, which emits /* #undef NAME */ when the substituted value is false-y. CMake counts 0 as false-y, so any x.y.0 release generates an installed header with that component silently missing. We're at v2.3.4-beta.2, so 2.4.0 would have tripped it.

• Plain #define for every macro the build unconditionally sets. cmakedefine stays only for ALPS_XML_ALTERNATE_DIR, which the build never sets and parser/xslt_path.C guards with #ifdef.
• Add ALPS_VERSION_NUMBER / ALPS_VERSION_NUM() for preprocessor version checks, packed like BOOST_VERSION.
• Drop ALPS_SVN_REVISION (expanded @ALPS_WC_REVISION@, unset since the SVN migration, always #undef) and ALPS_SRCDIR (baked the build machine's source path into an installed header for one line of pconfig output).

Verified by generating the header at 2.4.0 and 2.3.4 and compiling against each. set(ALPS_SRCDIR ...) is left in CMakeLists.txt on purpose — that file is contested by #107/#110/#112, and the follow-up rewrites the block.

Refs #95. Scoped to the two files no other open PR touches, so it can merge independently of the #107 → #111 → #112 stack.

[skilledwolf]

Folding this into #114, which single-sources the version from ALPS_VERSION.txt. The commit here is preserved as the first of the two commits there, so nothing is lost.

Rationale: on its own this PR was a 2-file fix with no user-visible change, and the set(ALPS_SRCDIR ...) it orphaned in CMakeLists.txt had to be left dangling for a follow-up. #114 removes that in the same change and pairs the header fix with the version plumbing it belongs to.

Branch fix/version-header-macros left in place for now; safe to delete.