Most of these issues are in autoconf-generated code and have already
been fixed upstream.  The remaining issue has been submitted as:

  <https://sourceforge.net/p/blt/patches/36/>

diff --git a/configure b/configure
index 3b7d5a3b30eeaa2a..25e968d1c0687800 100755
--- a/configure
+++ b/configure
@@ -975,7 +975,7 @@ cat > conftest.$ac_ext << EOF
 #line 976 "configure"
 #include "confdefs.h"
 
-main(){return(0);}
+int main(){return(0);}
 EOF
 if { (eval echo configure:981: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
   ac_cv_prog_cc_works=yes
@@ -1660,8 +1660,8 @@ else
 #define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))
 #define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
 int main () { int i; for (i = 0; i < 256; i++)
-if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) exit(2);
-exit (0); }
+if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) return 2;
+return 0; }
 
 EOF
 if { (eval echo configure:1668: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2371,12 +2371,12 @@ else
 #line 2372 "configure"
 #include "confdefs.h"
 #include <stdio.h>
-main()
+int main()
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(int));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2383: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2410,12 +2410,12 @@ else
 #line 2411 "configure"
 #include "confdefs.h"
 #include <stdio.h>
-main()
+int main()
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(long));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2422: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2449,12 +2449,12 @@ else
 #line 2450 "configure"
 #include "confdefs.h"
 #include <stdio.h>
-main()
+int main()
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(long long));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2461: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2488,12 +2488,12 @@ else
 #line 2489 "configure"
 #include "confdefs.h"
 #include <stdio.h>
-main()
+int main()
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(void *));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2500: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -3031,7 +3031,7 @@ if test "$ac_x_libraries" = NO; then
 cat > conftest.$ac_ext <<EOF
 #line 3033 "configure"
 #include "confdefs.h"
-
+char XtMalloc(void);
 int main() {
 ${x_direct_test_function}()
 ; return 0; }
diff --git a/configure.in b/configure.in
index 29600cbb868b0faa..7ea11f4c5ec8ecfe 100644
--- a/configure.in
+++ b/configure.in
@@ -350,7 +350,7 @@ if test "${blt_cv_found_dbl_epsilon}" = "no" ; then
   CFLAGS="-g -lm"
   AC_MSG_CHECKING([whether DBL_EPSILON can be computed])
   AC_TRY_RUN_WITH_OUTPUT(blt_cv_dbl_epsilon, [
-main () {
+int main () {
     double e, u;
     /* 
      *	Check the smallest value such that 1.0 + x != 1.0.