Always compile sources with known language

Refactoring by commit 11d9b211 (Add cmGeneratorTarget to represent a
target during generation, 2012-03-07) and commit 45c2f932 (Simplify
cmMakefileTargetGenerator using cmGeneratorTarget, 2012-03-07) preserved
behavior introduced by commit 7740ccd1 (some cleanup of the makefile
generator, 2006-02-14) that favored the IgnoreFile extension test over
the availability of a known compilation language associated with a
source file.  If a source is not marked as HEADER_FILE_ONLY and has a
known language extension or an explicit LANGUAGE property it should be
treated as that language.  The LANGUAGE source file property
documentation says so.