adding a build tag specific cgo bridge

[traetox]

Purpose of this PR

I am attempting to do some static linking and also to use an alternative LIBC implementation (musl) this requires that I have greater control over the LDFLAGS and CFLAGS. The current flags hard code the location of the library and include files, requiring some file copypasta.

What this PR does

Adds an optional customlib build tag that when specified during builds requires that the user provide all the CFLAG and LDFLAG variables.

What this allows

I can build the upstream wasmerio/wasmer package with custom targets, etc.. and then build my golang application with total control over which wasmer library file is included.

Example Usage

Assuming that i have cloned the wasmer repo to /wasmer and built an appropriate wasmer C-API I can then perform the following to get a sweet sweet statically compiled executable using the MUSL libc:

export CGO_CFLAGS="-I/wasmer/lib/c-api/"
export CGO_LDFLAGS="-Wl,-rpath,/wasmer/target/x86_64-unknown-linux-musl/release/ -L/wasmer/target/x86_64-unknown-linux-musl/release/ -pthread -lwasmer_c_api -lm -ldl -static"
CC=/usr/bin/musl-gcc go build -tags customlib

The additional build tag will not affect existing users at all.

[Hywan]

I like the idea very much, thanks!

I wonder whether customlib is a common name in the Go landscape?

[Hywan]

bors try

[bors]

try

Build succeeded:

• Build and Test (1.14, linux-amd64, ubuntu-18.04)
• Build and Test (1.14, linux-aarch64, self-hosted, linux, ARM64)
• Build and Test (1.15, linux-amd64, ubuntu-18.04)
• Build and Test (1.14, darwin-amd64, macos-latest)
• Build and Test (1.15, linux-aarch64, self-hosted, linux, ARM64)
• Build and Test (1.15, darwin-amd64, macos-latest)