compileflags and cflags disaster

[Kojoley]

compileflags is documented but ignored by every toolset except cray

b2/src/tools/features/compileflags-feature.jam

Lines 10 to 13 in f10c1d2

	[[bbv2.builtin.features.compileflags]]`compileflags`::
	The value of this feature is passed without modification to the corresponding
	tools. The values from the `compileflags` is applied to all compilation of any
	language for the tools.

cflags is documented and implemented in different places differently

b2/src/tools/features/cflags-feature.jam

Lines 10 to 15 in f10c1d2

	[[bbv2.builtin.features.cflags]]`cflags`; `cxxflags`; `linkflags`::
	The value of these features is passed without modification to the corresponding
	tools. For `cflags` that is both the C and {CPP} compilers, for `cxxflags` that
	is the {CPP} compiler, and for `linkflags` that is the linker. The features are
	handy when you are trying to do something special that cannot be achieved by a
	higher-level feature in B2.

b2/src/tools/gcc.jam

Lines 43 to 45 in f10c1d2

	`cflags`::
	Specifies additional compiler flags that will be used when compiling C
	sources.

and while in GCC it is documented to only apply to C sources it also applies to C++ sources, see also boostorg/build#548

When cflags applies to C++ sources too there is no way to require some flags only for C sources. I don't know which solution is better but at least it should be consistent, and some sort of C-only flags feature is needed. Either cflags becomes C-only and cfamilyflags is added if needed, or cflags is consistently becomes C and C++ flags and conlyflags is added to solve C-only flags need.