cc: use '-x c*-header' for header files for C and C++ linters. (#4318)

When linting an header file in C or C++, `-x c-header` or
`-x c++-header` should be used instead of `-x c` or `-x c++`.

Using `-x c` or `-x c++` for headers files can lead to unused variables
and functions marked as static inlined as seen in #4096.

Using `-x c-header` or `-x c++-header` solve these issues.

The list of file extensions that are considered as header files can be
configured with the variables `g:ale_c_cc_header_exts` and
`g:ale_cpp_cc_header_exts`.

doc/ale-c.txt

@@ -136,6 +136,17 @@ g:ale_c_cc_options                                         *g:ale_c_cc_options*
   This variable can be change to modify flags given to the C compiler.
 
 
+g:ale_c_cc_header_exts                                 *g:ale_c_cc_header_exts*
+                                                       *b:ale_c_cc_header_exts*
+  Type: |List|
+  Default: `['h']`
+
+  This variable can be change to modify the list of extensions of the files
+  considered as header files.
+
+  ALE will use `'-x c-header'` instead of `'-x c'` for header files.
+
+
 ===============================================================================
 ccls                                                               *ale-c-ccls*
 

doc/ale-cpp.txt

@@ -65,6 +65,17 @@ g:ale_cpp_cc_options                                     *g:ale_cpp_cc_options*
   This variable can be change to modify flags given to the C++ compiler.
 
 
+g:ale_cpp_cc_header_exts                             *g:ale_cpp_cc_header_exts*
+                                                     *b:ale_cpp_cc_header_exts*
+  Type: |List|
+  Default: `['h', 'hpp']`
+
+  This variable can be change to modify the list of extensions of the files
+  considered as header files.
+
+  ALE will use `'-x c++-header'` instead of `'-x c++'` for header files.
+
+
 ===============================================================================
 ccls                                                             *ale-cpp-ccls*