Avoid calling the undeclared exit function.  Include <string.h> for
strcmp.  Improves compatibility with future compilers which are likely
to reject implicit function declarations by default.

Submitted upstream:

  <https://github.com/performancecopilot/pcp/pull/1727>

diff --git a/configure b/configure
index d8726f045b264191..f285191756825493 100755
--- a/configure
+++ b/configure
@@ -14409,7 +14409,7 @@ main ()
 {
 
     double x = 123.456;
-    if (fpclassify(x) == FP_NAN) exit(1);
+    if (fpclassify(x) == FP_NAN) return 1;
 
   ;
   return 0;
@@ -14438,7 +14438,7 @@ main ()
 {
 
     double x = 123.456;
-    if (fpclassify(x) == FP_NAN) exit(1);
+    if (fpclassify(x) == FP_NAN) return 1;
 
   ;
   return 0;
@@ -16433,6 +16433,7 @@ else
 /* end confdefs.h.  */
 
 #include <time.h>
+#include <string.h>
 int main () {
     char b[32]="";
     time_t t = time(NULL);
diff --git a/configure.ac b/configure.ac
index f12e652dd5e6f20e..4f737c78ff63e670 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3097,7 +3097,7 @@ AC_TRY_LINK(
 ],
 [
     double x = 123.456;
-    if (fpclassify(x) == FP_NAN) exit(1);
+    if (fpclassify(x) == FP_NAN) return 1;
 ], ac_cv_func_fpclassify=yes)
 AC_MSG_RESULT($ac_cv_func_fpclassify)
 if test $ac_cv_func_fpclassify = no
@@ -3112,7 +3112,7 @@ then
 ],
 [
     double x = 123.456;
-    if (fpclassify(x) == FP_NAN) exit(1);
+    if (fpclassify(x) == FP_NAN) return 1;
 ], ac_cv_func_fpclassify=yes)
     AC_MSG_RESULT($ac_cv_func_fpclassify)
     if test $ac_cv_func_fpclassify = yes
@@ -3737,6 +3737,7 @@ AC_MSG_CHECKING([if strftime knows about %z])
 AC_TRY_RUN(
 [
 #include <time.h>
+#include <string.h>
 int main () {
     char b[32]="";
     time_t t = time(NULL);