Build picks up headers from system-wide slang install

[povik]

Reported over email: a build conflict between the vendored and system installs of slang

In file included from src/async_pattern.cc:9:
build/slang_install/include/slang/ast/expressions/ConversionExpression.h:15:20: error: redefinition of ‘class slang::ast::ConversionExpression’
   15 | class SLANG_EXPORT ConversionExpression : public Expression {
      |                    ^~~~~~~~~~~~~~~~~~~~
In file included from src/async_pattern.cc:8:
...system install path... /include/slang/ast/expressions/AssignmentExpressions.h:101:20: note: previous definition of ‘class slang::ast::ConversionExpression’
  101 | class SLANG_EXPORT ConversionExpression : public Expression {
      |                    ^~~~~~~~~~~~~~~~~~~~
In file included from ...system install path.../include/slang/util/Util.h:17,
                 from ...system install path.../include/slang/ast/HierarchicalReference.h:14,
                 from ...system install path.../include/slang/ast/Lookup.h:10,
                 from ...system install path.../include/slang/ast/ASTContext.h:12,
                 from ...system install path.../include/slang/ast/Expression.h:10,
                 from ...system install path.../include/slang/ast/TimingControl.h:10,
                 from src/async_pattern.cc:7:

I think a good clue is ConversionExpression.h is a header recently added to slang, the vendored slang version has it, but the system one probably doesn't in this case.