curl-7.50.2 introduced a #define without integer value:

    #ifdef CURL_NO_OLDIES
    #define CURL_STRICTER
    #endif

Heuristics in 'Makefile.PL' assumes all defines in form of
    #define CURL_<something> <an-expression>
and generates a symbol lookup table in 'curlopt-constants.c'
as:

  static int
  constant(const char *name)
  {
    errno = 0;

    if (strncmp(name, "CURL_", 5) == 0) {
        name += 5;
        switch (*name) {
        ...
        case 'S':
            if (strEQ(name, "STRICTER")) return CURL_STRICTER;
Which is not valid C:
  curlopt-constants.c:128:49: error: ‘CURL_STRICTER’ undeclared (first use in this function)
             if (strEQ(name, "STRICTER")) return CURL_STRICTER;
                                                 ^~~~~~~~~~~~~
diff --git a/Makefile.PL b/Makefile.PL
index f9170bb..fc1a55a 100644
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -122,2 +122,9 @@ if (!defined($curl_h)) {
     while(<H>) {
+        # Skip defines without values like:
+        #    #define CURL_STRICTER
+        if (/^#define (CURL[A-Za-z0-9_]*)$/) {
+            chomp;
+            warn "Skipping '$_': does not define a symbol";
+            next;
+        }
         if (/^#define (CURL[A-Za-z0-9_]*)/) {