cross-platform: Improve clang error reporting with build.sh

Fixes #954

build.sh

@@ -110,14 +110,20 @@ if [[ ${#_VERBOSE} > 0 ]]; then
     echo ""
 fi
 
+CLANG_PATH=
 if [[ ${#_CXX} > 0 || ${#_CC} > 0 ]]; then
+    if [[ ${#_CXX} == 0 || ${#_CC} == 0 ]]; then
+        echo "ERROR: '-cxx' and '-cc' options must be used together."
+        exit 1
+    fi
     echo "Custom CXX ${_CXX}"
     echo "Custom CC  ${_CC}"
 
     if [[ ! -f $_CXX || ! -f $_CC ]]; then
         echo "ERROR: Custom compiler not found on given path"
         exit 1
     fi
+    CLANG_PATH=$_CXX
 else
     RET_VAL=$(SAFE_RUN 'c++ --version')
     if [[ ! $RET_VAL =~ "clang" ]]; then
@@ -126,16 +132,28 @@ else
             echo "Clang++ found at /usr/bin/clang++"
             _CXX=/usr/bin/clang++
             _CC=/usr/bin/clang
+            CLANG_PATH=$_CXX
         else
             echo "ERROR: clang++ not found at /usr/bin/clang++"
             echo ""
             echo "You could use clang++ from a custom location."
             PRINT_USAGE
             exit 1
         fi
+    else
+        CLANG_PATH=c++
     fi
 fi
 
+# check clang version (min required 3.7)
+VERSION=$($CLANG_PATH --version | grep "version [0-9]*\.[0-9]*" --o -i | grep "[0-9]\.[0-9]*" --o)
+VERSION=${VERSION/./}
+
+if [[ $VERSION -lt 37 ]]; then
+    echo "ERROR: Minimum required Clang version is 3.7"
+    exit 1
+fi
+
 CC_PREFIX=""
 if [[ ${#_CXX} > 0 ]]; then
     CC_PREFIX="-DCMAKE_CXX_COMPILER=$_CXX -DCMAKE_C_COMPILER=$_CC"