cmake: FindClang: accept 3.7

[azat]

Otherwise

/usr/bin/ld: warning: libclangRewriteFrontend.so.3, needed by llvm/.cmake-v2/lib/libclangFrontendTool.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld: warning: libclangASTMatchers.so.3, needed by llvm/.cmake-v2/lib/libclangTooling.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld: warning: libclangToolingCore.so.3, needed by llvm/.cmake-v2/lib/libclangTooling.so, not found (try using -rpath or -rpath-link)
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::MatchFinder::MatchCallback::~MatchCallback()'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `vtable for clang::RewriteTestAction'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::tooling::Replacement::Replacement(clang::SourceManager const&, clang::CharSourceRange const&, llvm::StringRef)'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `vtable for clang::ast_matchers::MatchFinder::MatchCallback'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::internal::DynTypedMatcher::constructVariadic(clang::ast_matchers::internal::DynTypedMatcher::VariadicOperator, std::vector<clang::ast_matchers::internal::DynTypedMatcher, std
::allocator<clang::ast_matchers::internal::DynTypedMatcher> >)'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::tooling::applyAllReplacements(std::set<clang::tooling::Replacement, std::less<clang::tooling::Replacement>, std::allocator<clang::tooling::Replacement> > const&, clang::Rewriter&)'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `vtable for clang::FixItRecompile'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::internal::DynTypedMatcher::dynCastTo(clang::ast_type_traits::ASTNodeKind) const'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `vtable for clang::RewriteMacrosAction'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::tooling::operator<(clang::tooling::Replacement const&, clang::tooling::Replacement const&)'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::MatchFinder::MatchCallback::getID() const'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `clang::FixItAction::FixItAction()'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `vtable for clang::RewriteIncludesAction'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::internal::BoundNodesTreeBuilder::addMatch(clang::ast_matchers::internal::BoundNodesTreeBuilder const&)'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::internal::HasNameMatcher::HasNameMatcher(llvm::StringRef)'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::internal::DynTypedMatcher::matches(clang::ast_type_traits::DynTypedNode const&, clang::ast_matchers::internal::ASTMatchFinder*, clang::ast_matchers::internal::BoundNodesTreeB
uilder*) const'
llvm/.cmake-v2/lib/libclangTooling.so: undefined reference to `clang::ast_matchers::internal::DynTypedMatcher::trueMatcher(clang::ast_type_traits::ASTNodeKind)'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `vtable for clang::RewriteObjCAction'
llvm/.cmake-v2/lib/libclangFrontendTool.so: undefined reference to `vtable for clang::HTMLPrintAction'

Also there are some issues with command line options (conflicts, since llvm have some defaults from some point):

LLVM ERROR: inconsistency in registered CommandLine options

You could pick one of the following patches to fix this:
azat-archive@6361d6c # very bad idea
azat-archive@8def87a # -Wl,--as-needed

(Also I'm working on linking generator only with libs that used there, to avoid such hacks, but this will need more time).

[ogoffart]

Thanks, I also cherry-picked the -Wl,--as-needed change