[m-rev.] for review: Provide a helpful error if no C sharp compiler could be found

[Paul Bone]

When I was addressing the review comments of my earlier submitted patches, I
saw the opportunity to add the following additional change.

--

Provide a helpful error if no C sharp compiler could be found

m4/mercury.m4:
    If the user specified one or more possible C sharp compilers and none
    of them were suitable, then report an error and abort.
---
 m4/mercury.m4 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/m4/mercury.m4 b/m4/mercury.m4
index 88aa79f..dd34cc8 100644
--- a/m4/mercury.m4
+++ b/m4/mercury.m4
@@ -369,6 +369,13 @@ EOF
     esac
 done
 
+# If the user specified one or more compilers and we couldn't find any
+# then abort configuration.
+if test "$mercury_cv_with_csharp_compiler" != "" -a "$CSC" = ""; then
+    AC_MSG_ERROR([No C sharp compilers could be found.])
+    exit 1
+fi
+
 case "$CSC" in
     csc*)
         CSHARP_COMPILER_TYPE=microsoft
-- 
1.8.1.3