build: Append existing CFLAGS and LDFLAGS for CGO

Underlying `go build` commands can pass custom `CFLAGS` and `LDFLAGS` to
CGO invocations, which GoPy uses in order to pass some of its own
command-line arguments to builds.

However, there are cases where additional, build-specific parameters may
need to be used (e.g. in the case where we're linking against a static
library which itself links to other dynamic libraries, which aren't set
as options in the packages themselves); this commit respects any existing
uses of the `CGO_CFLAGS` and `CGO_LDFLAGS` environment variables, and
appends their values to ones used internally by GoPy if needed.

Author: deuill

cmd_build.go

@@ -235,6 +235,9 @@ func runBuild(mode bind.BuildMode, cfg *BuildCfg) error {
 		if include, exists := os.LookupEnv("GOPY_INCLUDE"); exists {
 			cflags = append(cflags, "-I"+filepath.ToSlash(include))
 		}
+		if oldcflags, exists := os.LookupEnv("CGO_CFLAGS"); exists {
+			cflags = append(cflags, oldcflags)
+		}
 		var ldflags []string
 		if cfg.DynamicLinking {
 			ldflags = strings.Fields(strings.TrimSpace(pycfg.LdDynamicFlags))
@@ -250,6 +253,9 @@ func runBuild(mode bind.BuildMode, cfg *BuildCfg) error {
 		if libname, exists := os.LookupEnv("GOPY_PYLIB"); exists {
 			ldflags = append(ldflags, "-l"+filepath.ToSlash(libname))
 		}
+		if oldldflags, exists := os.LookupEnv("CGO_LDFLAGS"); exists {
+			ldflags = append(ldflags, oldldflags)
+		}
 
 		removeEmpty := func(src []string) []string {
 			o := make([]string, 0, len(src))