PKG_CONFIG_ALLOW_SYSTEM_CFLAGS and cross-compiling

[plauche]

Hi!

I recently ran into a very strange cross-compiling issue and tracked it down to the changes in #93. My crate depends on other crates which use pkg-config to locate system libraries. Things are all well and good in the native build environment, however having this flag enabled by default is causing me grief when cross-compiling.

My linker is not too keen on having /usr/include passed along as an include folder:

cargo:warning=arm-buildroot-linux-gnueabihf-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I' '/usr/include'

The changelog says that PGK_CONFIG_ALLOW_SYSTEM_CFLAGS is enabled by default, so I'm assuming that means there is a way to disable it.

My question - Can I disable this via configuration in Cargo.toml?
I don't have direct access to the crates which are actually using pkg-config for library location.

Currently I'm just pinning this crate to 0.3.15 in one of my Cargo.toml files so that 0.3.16 is not used.