Check*CompilerFlag: Avoid ';' in common pattern (#15048)

In commit v3.0.0-rc1~551^2~1 (Check*CompilerFlag: make C and CXX modules
share most error patterns, 2013-08-08) a pattern containing a ';' was
moved out of a ""-quoted argument and into a variable.  CMake flattens
the containing list and breaks the pattern.  Use a '.' to match ';'.
Merge branch 'check-flag-avoid-semicolon' into release