MSVC requires that the C PCH file is compiled (as an object) and linked if
any C file is found, and the same for C++.
Most qmake projects are C++.
If a C++ project has a precompiled header, it is typically of C++ type, and
cannot be compiled as C (for example, it contains or includes classes).
Since there is no easy way to conditionally build the C PCH file only if C
files are found in the project (as done for g++), we need a setting that is
disabled by default.
This amends 30331afda1.
[ChangeLog][Tools][qmake] Introduced precompile_header_c CONFIG option for
MSVC to enable precompiled header for C sources.
Task-number: QTBUG-65103
Change-Id: Id9688a35ee7d9b5e4f5a846b81986cb674bc5f4e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
af1a179cc8