v help: describe CFLAGS and LDFLAGS support in v help build-c

Author: spytheman

cmd/v/help/build-c.txt

@@ -16,7 +16,13 @@ see also `v help build`.
       Pass the provided flag as is to the C compiler.
       Can be specified multiple times to provide multiple flags.
       Use quotes to wrap the flag argument if it contains spaces.
-      
+
+      V also supports the environment variables CFLAGS and LDFLAGS.
+      The contents of the CFLAGS variable will be prepended as is, at the start
+      of the C backend command, right after the name of the compiler.
+      The contents of the LDFLAGS variable will be appended as is, at the end
+      of the C backend command, after all other options.
+
    -cstrict
       Turn on additional C warnings. This slows down compilation
       slightly (~10-10% for gcc), but sometimes provides better diagnosis.