configure.ps1: do not test SpiderMonkey libs when it is disabled

When Windows users decide to disable SpiderMonkey, it is likely
the case they do not have it installed either.  In those scenarios
it makes no sense to check for the presence of the static
libraries.  Instead, `configure.ps1` fails to proceed untruly,
which has to be fixed.

Author: pgj

configure.ps1

@@ -123,20 +123,22 @@ If ($Test) {
     exit 0
 }
 
-# Use the MSVC linker to determine if the respective SpiderMonkey library
-# is available on the linker path.  This heuristic is taken from
-# src/couch/rebar.config.script, please keep them in sync.
-If ($SpiderMonkeyVersion -eq "1.8.5") {
-    $SpiderMonkeyLib = "mozjs185-1.0.lib"
-}
-else {
-    $SpiderMonkeyLib = "mozjs-$SpiderMonkeyVersion.lib"
-}
+If (-Not $DisableSpiderMonkey) {
+    # Use the MSVC linker to determine if the respective SpiderMonkey library
+    # is available on the linker path.  This heuristic is taken from
+    # src/couch/rebar.config.script, please keep them in sync.
+    If ($SpiderMonkeyVersion -eq "1.8.5") {
+	$SpiderMonkeyLib = "mozjs185-1.0.lib"
+    }
+    else {
+	$SpiderMonkeyLib = "mozjs-$SpiderMonkeyVersion.lib"
+    }
 
-&link $SpiderMonkeyLib /SUBSYSTEM:CONSOLE /NOENTRY /DLL /OUT:NUL *> $null
-If ($LASTEXITCODE -ne 0) {
-    Write-Output "ERROR: SpiderMonkey $SpiderMonkeyVersion is not found. Please specify with -SpiderMonkeyVersion."
-    exit 1
+    &link $SpiderMonkeyLib /SUBSYSTEM:CONSOLE /NOENTRY /DLL /OUT:NUL *> $null
+    If ($LASTEXITCODE -ne 0) {
+	Write-Output "ERROR: SpiderMonkey $SpiderMonkeyVersion is not found. Please specify with -SpiderMonkeyVersion."
+	exit 1
+    }
 }
 
 # Translate ./configure variables to CouchDB variables