Add bb_full_target within the build environment. (#644)

Closes #619

docs/src/build_tips.md

@@ -133,6 +133,7 @@ The following variables are useful to control the build script over different ta
 * `libdir`: the path to the directory where the shared libraries should be installed.  This is `${prefix}/bin` when building for Windows, `${prefix}/lib` for all other platforms
 * `bindir`: the path to the directory where the executables should be installed.  This is equivalent to `${prefix}/bin`
 * `target`: the target platform
+* `bb_full_target`: the full target platform, containing things like libstdc++ string ABI platform tags, and libgfortran version
 * `nproc`: the number of processors of the host machine, useful for parallel building (e.g., `make -j${nproc}`)
 * `nbits`: number of bits of the target architecture (usually it is either 32 or 64)
 * `proc_family`: target processor family (e.g., "intel", "power", or "arm")

src/Runner.jl

@@ -506,6 +506,8 @@ function platform_envs(platform::Platform, src_name::AbstractString; host_platfo
         # overwritten in `./configure`, and we want tools like `uname` to still see it)
         "bb_target" => target,
         "target" => target,
+        # Also give people access to the full `-cxxYY` and `-libgfortranX` triplet if they really want it
+        "bb_full_target" => triplet(platform),
         "rust_target" => map_rust_target(platform),
         "rust_host" => map_rust_target(rust_host), # use glibc since musl is broken. :( https://github.com/rust-lang/rust/issues/59302
         "nproc" => "$(get(ENV, "BINARYBUILDER_NPROC", Sys.CPU_THREADS))",