Add enviroment variable to disable default flags: CRATE_CC_NO_DEFAULTS

Author: GeorgeHahn

README.md

@@ -77,6 +77,7 @@ number of different environment variables.
          certain `TARGET`s, it also is assumed to know about other flags (most
          common is `-fPIC`).
 * `AR` - the `ar` (archiver) executable to use to build the static library.
+* `CRATE_CC_NO_DEFAULTS` - the default compiler flags may cause conflicts in some cross compiling scenarios. Setting this variable will disable the generation of default compiler flags.
 
 Each of these variables can also be supplied with certain prefixes and suffixes,
 in the following prioritized order: