Always add command path to environment in GccToolchain.

Fixes: SOFT ASSERT: "predefinedMacros.startsWith("#define ")" in
file .\gcctoolchain.cpp for MinGW since the g++ - invocation
to determine the macros causes g++ to launch a subprocess
which then fails.

Change-Id: If4e7b302ac57f7cbed4982a468bcb1cb3b39f028
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Output warning when g++ - invocation fails in GccToolchain.

Change-Id: I5ed05307531f0d3a7850fb251cfa5cbd2a75e09f
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>