GenerateExportHeader: Avoid if() quoted auto-dereference

When testing CMAKE_CXX_COMPILER_ID values with if(MATCHES),
do not explicitly dereference or quote CMAKE_CXX_COMPILER_ID.
We want if() to auto-dereference the variable and not its value.
Merge topic 'cleanup-CMP0054' into next

69fe5920 GenerateExportHeader: Avoid if() quoted auto-dereference