Allow configure to select valac version; fix #6704

We recognize the VALAC environment variable, or you can specify
VALAC=whatever on the ./configure command line.

Also, this cleans up some of the configure logic to be a little easier
to read and edit.

configure

@@ -10,32 +10,32 @@ DEFAULT_PREFIX="/usr/local"
 PREFIX=$DEFAULT_PREFIX
 
 configure_help() {
-    printf "Usage:\n"
-    printf "\t./configure [OPTIONS]...\n"
-    printf "\n"
-    printf "Options:\n"
-    
-    printf "\t-h, --help\t\tPrint this help and exit.\n"
-    
-    printf "\t--prefix=PREFIX\t\tPrepend PREFIX to program installation paths.\n"
-    printf "\t\t\t\t[%s]\n" $DEFAULT_PREFIX
-    
-    printf "\t--debug\n"
-    printf "\t\t\t\tBuild for debugging.\n"
-    
-    printf "\t--enable-ref-tracking\n"
-    printf "\t\t\t\tEnable reference tracking which is dumped to stdout when the program exits.\n"
-    
-    printf "\t--disable-schemas-compile\n"
-    printf "\t\t\t\tDisable compiling the GSettings schema.\n"
-    
-    printf "\t--disable-desktop-update\n"
-    printf "\t\t\t\tDisable desktop database update.\n"
-    
-    printf "\t--disable-icon-update\n"
-    printf "\t\t\t\tDisable icon cache update.\n"
-    
-    printf "\n"
+    cat <<- EOT
+	Usage:
+	  ./configure [OPTIONS]...
+	
+	Options:
+	  -h, --help            Print this help and exit.
+	  --prefix=PREFIX       Prepend PREFIX to program installation paths.
+	                        [$DEFAULT_PREFIX]
+	  --debug               Build for debugging.
+
+	  --enable-ref-tracking
+	                        Enable reference tracking which is dumped to stdout when the program exits.
+	  --disable-schemas-compile
+	                        Disable compiling the GSettings schema.
+	  --disable-desktop-update
+	                        Disable desktop database update.
+	  --disable-icon-update
+	                        Disable icon cache update.
+
+	Some influential environment variables:
+	  PKG_CONFIG_PATH       Adds directories to pkg-config's search path.
+	  PKG_CONFIG_LIBDIR     Overrides pkg-config's built-in search path.
+
+	  VALAC                 Name of the vala compiler to use, e.g. "valac-0.18".
+
+EOT
 }
 
 abort() {
@@ -46,10 +46,13 @@ abort() {
 
 while [ $# != 0 ]
 do
-    option=`echo $1 | sed 's/=.*//'`
-    if [ `echo $1 | grep '='` ]
+    if [[ "$1" = *=* ]]
     then
-        value=`echo $1 | sed 's/.*=//'`
+        option=${1%%=*}
+        value=${1#*=}
+    else
+        option=$1
+        value=
     fi
 
     case $option in
@@ -57,11 +60,7 @@ do
                             exit 0
                             ;;
         
-        --prefix)           if [ ! $value ]
-                            then
-                                abort $1
-                            fi
-                            
+        --prefix)           [ ! $value ] && abort $1
                             CMDLINE="${CMDLINE} -DCMAKE_INSTALL_PREFIX=${value}"
                             ;;
         
@@ -86,6 +85,10 @@ do
                             CMDLINE="${CMDLINE} -DDESKTOP_UPDATE=OFF"
                             ;;
         
+        VALAC)              [ ! $value ] && abort $1
+                            VALAC=$value
+                            ;;
+        
         *)                  abort $option
                             ;;
     esac
@@ -93,6 +96,20 @@ do
     shift
 done
 
+# Verify use supplied vala executable
+if [ -n "$VALAC" ]
+then
+    VALA_EXECUTABLE=`type -p "$VALAC"`
+
+    if [ -z "$VALA_EXECUTABLE" ]
+    then
+        printf "$VALAC is not an executable program.\n"
+        exit 1
+    fi
+
+    CMDLINE="${CMDLINE} -DVALA_EXECUTABLE='$VALA_EXECUTABLE'"
+fi
+
 # Verify cmake is installed
 # TODO: Check for minimum version number
 if ! cmake --version