Allow overriding of compiler in build-qt job

Evaluate --cc and --cxx parameters, default to the highest installed
version of gcc or clang.

minicoin/jobs/build-qt/help.txt

@@ -28,7 +28,7 @@ Supported job parameters:
 
   Override the default build tree:
 
-  $minicoin run ubuntu1804 build-qt -- --build qt-build-5.15
+  $ minicoin run ubuntu1804 build-qt -- --build qt-build-5.15
 
 --clean
 
@@ -39,3 +39,10 @@ Supported job parameters:
   Use  qmake build system also if a cmake build system is present.
   Ignored if the build is already configured with cmake, combine with
   --clean or use a separate --build to switch build systems.
+
+--cc path
+--cxx path
+
+  Use the C and C++ compiler at the specified path, e.g.
+
+  $ minicoin run ubuntu1804 build-qt -- -cc /usr/bin/clang-10 --cxx /usr/bin/clang++10

minicoin/jobs/build-qt/main.sh

@@ -7,6 +7,23 @@ then
   exit 1
 fi
 
+# use highest installed version of gcc or clang, unless compilers are specified
+function search_highest
+{
+  [ -z $1 ] && return
+  local search_version=8
+  local highest_found=
+  while [[ -f "${1}-${search_version}" ]]
+  do
+    highest_found=$search_version
+    search_version=$(( $search_version + 1 ))
+  done
+  [ -z highest_found ] || echo ${1}-${highest_found}
+}
+
+export CC=${PARAM_cc:-$(search_highest "$(which gcc || which clang)")}
+export CXX=${PARAM_cxx:-$(search_highest "$(which g++ || which clang++)")}
+
 # set defaults
 build_dir=${PARAM_build:-"qt-build"}
 target=$PARAM_target