Improve how gcc/clang is detected in Makefiles

Rather than check if $(CC) IS gcc or IS clang check for $(CC) containing
gcc or clang. This is because in some systems or conditions the compiler
might not be called JUST gcc or JUST clang. For instance gcc in MacPorts
is called gcc-mp-13 (for example) but without this change one would have
to symlink gcc to gcc-mp-13 or copy it to gcc.

The FAQ has been updated to address this.

faq.md

@@ -883,11 +883,14 @@ While we know that use of `-Weverything` is generally not recommended
 by `clang` C compiler developers, we do use the `-Weverything`
 option in a certain case in IOCCC winner `Makefile`s.
 
-The use of `-Weverything` is limited to when one forces `CC=clang`. Users with
-clang compilers are not required to set `CC=clang` but when they do,
-`-Weverything` is enabled with all of its challenges, pedantic warnings, and
-sometimes warnings about things that do not matter, some of which are frankly
-frivolous and often downright dubious.
+The use of `-Weverything` is limited to when one forces `CC=clang` or more
+generally if the string 'clang' is in `$(CC)`. If `$(CC)` contains 'gcc' the
+'gcc' specific options are run instead.
+
+Users with clang compilers are not required to set `CC=clang` or equivalent but
+when they do, `-Weverything` is enabled with all of its challenges, pedantic
+warnings, and sometimes warnings about things that do not matter, some of which
+are frankly frivolous and often downright dubious.
 
 To enable this feature: