This patch is a result of rake-compiler-dock using centos 7 (manylinux2014) to cross-compile.

Centos, for reasons I have not been able to discern, implements `isnan` and `isinf` as a function
and not as a macro. Debian knows how to resolve that function at dynamic-link time (despite using a
macro at compile time), but musl-based systems (like alpine) do not. Running `nm` on nokogiri.so
created on such a centos system shows:

```
                 U __isinf@@GLIBC_2.2.5
                 U __isnan@@GLIBC_2.2.5
```

(see https://github.com/sparklemotion/nokogiri/pull/2142 for more info)

This patch avoids using glibc's `isnan` and `isinf` calls, instead using libxml2's fallback
implementation. There's history here, see libxml2 commit 8813f39:

    commit 8813f39
    Author: Nick Wellnhofer <wellnhofer@aevum.de>
    Date:   2017-09-21 00:11:26 +0200
    
        Simplify XPath NaN, inf and -0 handling
        
        Use C99 macros NAN, INFINITY, isnan, isinf. If they're not available:
        
        - Assume that (0.0 / 0.0) generates a NaN and !(x == x) tests for NaN.
        - Use C89's HUGE_VAL for INFINITY.
        
        Remove manual handling of NaN, infinity and negative zero in functions
        xmlXPathValueFlipSign and xmlXPathDivValues.
        
        Remove xmlXPathGetSign. All the tests for negative zero can be replaced
        with a test for negative or positive zero.
        
        Simplify xmlXPathRoundFunction.
        
        Remove Trio dependency.
        
        This should work on IEEE 754 compliant implementations even if the C99
        macros aren't available, but will likely break some ancient platforms.
        If problems arise, my plan is to port the relevant trionan.c solution
        to xpath.c. Note that non-compliant implementations are impossible
        to fully support, anyway, since XPath requires IEEE 754.

This patch would be unnecessary if any of the following was true:

* centos implements these as macros, and doesn't generate an unresolved symbol for either in the shared library
* we had a way to ensure `__isinf` and `__isnan` resolve on musl (e.g., we implement them locally)

diff --git a/xpath.c b/xpath.c
index 9f64ab9..5b6d999 100644
--- a/xpath.c
+++ b/xpath.c
@@ -509,11 +509,7 @@ xmlXPathInit(void) {
  */
 int
 xmlXPathIsNaN(double val) {
-#ifdef isnan
-    return isnan(val);
-#else
     return !(val == val);
-#endif
 }
 
 /**
@@ -524,15 +520,11 @@ xmlXPathIsNaN(double val) {
  */
 int
 xmlXPathIsInf(double val) {
-#ifdef isinf
-    return isinf(val) ? (val > 0 ? 1 : -1) : 0;
-#else
     if (val >= INFINITY)
         return 1;
     if (val <= -INFINITY)
         return -1;
     return 0;
-#endif
 }
 
 #endif /* SCHEMAS or XPATH */