Clean up find_msms()

...incorporating insight documented in the commit note of
c9c2e27c "Adapt for Visual Studio 2013," and
cloph's idea to use Perl to read that oddly named Windows env var in cygwin.

Change-Id: I55d364f7a24b50d171a7fdca000ca2d747f6db20

configure.ac

@@ -5103,26 +5103,41 @@ find_winsdk()
 
 find_msms()
 {
+    my_msm_file=Microsoft_VC${VCVER}_CRT_x86.msm
+    AC_MSG_CHECKING([for $my_msm_file])
+    msmdir=
     for ver in 10.0 11.0 12.0; do
         reg_get_value_32 HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/VisualStudio/$ver/Setup/VS/MSMDir
         if test -n "$regvalue"; then
-            if test -e "$regvalue/Microsoft_VC${VCVER}_CRT_x86.msm"; then
+            if test -e "$regvalue/$my_msm_file"; then
                 msmdir=$regvalue
                 break
             fi
         fi
     done
+    dnl Is the following fallback really necessary, or was it added in response
+    dnl to never having started Visual Studio on a given machine, so the
+    dnl registry keys checked above had presumably not yet been created?
+    dnl Anyway, if it really is necessary, it might be worthwhile to extend it
+    dnl to also check %CommonProgramFiles(X86)% (typically expanding to
+    dnl "C:\Program Files (X86)\Common Files" compared to %CommonProgramFiles%
+    dnl expanding to "C:\Program Files\Common Files"), which would need
+    dnl something like $(perl -e 'print $ENV{"CommonProgramFiles(x86)"}') to
+    dnl obtain its value from cygwin:
     if test -z "$msmdir"; then
-        AC_MSG_NOTICE([no registry entry for Merge Module directory - trying "${COMMONPROGRAMFILES}/Merge Modules/"])
-        msmdir="${COMMONPROGRAMFILES}/Merge Modules/"
+        my_msm_dir="${COMMONPROGRAMFILES}/Merge Modules"
+        if test -e "$my_msm_dir/$my_msm_file"; then
+            msmdir=$my_msm_dir
+        fi
     fi
-    msmdir=`cygpath -m "$msmdir"`
-    if test -z "$msmdir"; then
+    if test -n "$msmdir"; then
+        msmdir=`cygpath -m "$msmdir"`
+        AC_MSG_RESULT([$msmdir])
+    else
         if test "$ENABLE_RELEASE_BUILD" = "TRUE" ; then
-            AC_MSG_ERROR([Merge modules not found in $msmdir])
+            AC_MSG_ERROR([not found])
         else
-            AC_MSG_WARN([Merge modules not found in $msmdir])
-            msmdir=""
+            AC_MSG_WARN([not found])
         fi
     fi
 }