component/golang: set gcc's path in workspace's PATH

It seems some programs can only be built by the same gcc than the one used to
build golang itself.

For example, when system gcc is gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516,
grafana-server fail to build with:

    /data/slappart11_testnode/cqg/inst/test0-0/tmp/shared/golang1.12/fbee59cfb3c995382cf70d409615aa54/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
    /usr/bin/ld: /tmp/go-link-305363633/000000.o: unable to initialize decompress status for section .debug_info
    /usr/bin/ld: /tmp/go-link-305363633/000000.o: unable to initialize decompress status for section .debug_info
    /usr/bin/ld: /tmp/go-link-305363633/000000.o: unable to initialize decompress status for section .debug_info
    /usr/bin/ld: /tmp/go-link-305363633/000000.o: unable to initialize decompress status for section .debug_info
    /tmp/go-link-305363633/000000.o: file not recognized: File format not recognized
    collect2: error: ld returned 1 exit status

Also generally, if we don't trust system gcc to build golang, we cannot trust
it to build golang programs.

The downside is that if components or software want to use a specifig
golang version they have to set both golang= and the corresponding
gcc-bin-directory= in their [gowork]

component/caddy/buildout.cfg

@@ -8,6 +8,8 @@ parts =
 [gowork]
 # Caddy 1.x+ uses go modules, for which gowork does not work yet
 golang  = ${golang1.12:location}
+gcc-bin-directory  = ${golang1.12:gcc-bin-directory}
+
 install =
 
 [gowork.goinstall]

component/golang/buildout.cfg

@@ -17,10 +17,12 @@ configure-command = :
 location = @@LOCATION@@
 make-binary =
 make-targets= cd src && ./all.bash && cp -alf .. ${:location}
+# gcc version to use
+gcc-bin-directory = ${gcc-8.2:location}/bin
 # some testdata files have an issue with slapos.extension.strip.
 post-install = ${findutils:location}/bin/find ${:location}/src -type d -name testdata -exec rm -rf {} \; || true
 environment =
-  PATH=${gcc-8.2:location}/bin:%(PATH)s
+  PATH=${:gcc-bin-directory}:%(PATH)s
   GOROOT_FINAL=${:location}
   ${:environment-extra}
 
@@ -104,6 +106,9 @@ depends = ${gowork.goinstall:recipe}
 # go version used for the workspace (possible to override in applications)
 golang  = ${golang1.10:location}
 
+# gcc version must be compatible with go version selected
+gcc-bin-directory = ${golang1.10:gcc-bin-directory}
+
 # no special build flags by default
 buildflags =
 
@@ -123,7 +128,7 @@ recipe	= slapos.recipe.template
 url     = ${:_profile_base_location_}/goenv.sh.in
 output	= ${gowork:directory}/env.sh
 depends = ${gowork.mkdir:recipe}
-md5sum	= 7a067a3974c446c3eaa0e82818ba1adb
+md5sum	= 7eaad1f9aabd3cfad554975098c5d4c3
 
 [gowork.mkdir]
 # NOTE do not use slapos.cookbook:mkdirectory here - if anything in software (not instance)

component/golang/goenv.sh.in

@@ -3,7 +3,7 @@
 
 # ---- 8< ---- (buildout substitution here)
 # PATH so that go & friends work out of the box
-export PATH=${gowork:golang}/bin:${git:location}/bin:${pkgconfig:location}/bin:${buildout:bin-directory}:$PATH
+export PATH=${gowork:golang}/bin:${git:location}/bin:${pkgconfig:location}/bin:${buildout:bin-directory}:${gowork:gcc-bin-directory}:$PATH
 X=${gowork:directory}
 
 export PKG_CONFIG_PATH=$(echo -n "${gowork:cpkgpath}" |tr '\n' ':'):$PKG_CONFIG_PATH

component/helloweb/buildout.cfg

@@ -26,7 +26,7 @@ install =
     lab.nexedi.com/nexedi/helloweb/go/...
 
 golang  = ${golang1.12:location}
-
+gcc-bin-directory  = ${golang1.12:gcc-bin-directory}
 
 # -*- go -*-
 [helloweb-go]