build: Error out early if flex or bison are not found.

The configure script tests for these executables, but we happily
proceed even if they are not found. For the m4 executable we got
the check right, however. Introduce a general helper that calls
AC_PATH_PROG() to locate an executable and aborts if it is not found.

diff --git a/configure.ac b/configure.ac
index 1a375e9f57b0c9d589b24caf7faf2fe4b02a861e..499571a70318f2d3bfe753e8a9be95307e7fc378 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -46,20 +46,21 @@ AC_DEFUN([LIB_SUBST_FLAGS], [
        AC_SUBST($1_cppflags)
        AC_SUBST($1_ldflags)
 ])
+AC_DEFUN([REQUIRE_EXECUTABLE], [
+        AC_PATH_PROG(m4_toupper([$1]), [$1])
+        test -z "$m4_toupper([$1])" && AC_MSG_ERROR(
+               [$1 is required to build this package])
+])
 
 AC_USE_SYSTEM_EXTENSIONS
 AC_C_BIGENDIAN()
-
-AC_PATH_PROG([BISON], [bison])
-AC_PATH_PROG([FLEX], [flex])
-
-AC_PATH_PROG([M4], [m4])
-test -z "$M4" && AC_MSG_ERROR(
-       [The m4 macro processor is required to build this package])
-
 AC_PROG_CC
 AC_PROG_CPP
 
+REQUIRE_EXECUTABLE([bison])
+REQUIRE_EXECUTABLE([flex])
+REQUIRE_EXECUTABLE([m4])
+
 executables="recv filter audioc write afh play"
 ########################################################################### osl
 STASH_FLAGS