build: make waf append pkgconfig flags as-is

waf apparently only appends a pkgconfig flag if it doesn't already exist in
the lib storage. Since our configure often checks for multiple libraries in
one call we want to keep the flags as is. This is especially important to
always keep stuff like -lm in the right place.

waftools/checks/generic.py

@@ -1,5 +1,6 @@
 import os
 from inflectors import DependencyInflector
+from waflib.ConfigSet import ConfigSet
 
 __all__ = [
     "check_pkg_config", "check_cc", "check_statement", "check_libs",
@@ -66,12 +67,22 @@ def check_pkg_config(*args, **kw_ext):
             'package': " ".join(packages),
             'args':    sargs + pkgc_args }
         opts = __merge_options__(dependency_identifier, defaults, kw_ext, kw)
-        if ctx.check_cfg(**opts):
-            return True
-        else:
+
+        # Warning! Megahack incoming: when parsing flags in `parse_flags` waf
+        # uses append_unique. This appends the flags only if they aren't
+        # already present in the list. This causes breakage if one checks for
+        # multiple pkg-config packages in a single call as stuff like -lm is
+        # added only at its first occurrence.
+        original_append_unique  = ConfigSet.append_unique
+        ConfigSet.append_unique = ConfigSet.append_value
+        result = bool(ctx.check_cfg(**opts))
+        ConfigSet.append_unique = original_append_unique
+
+        if not result:
             defkey = DependencyInflector(dependency_identifier).define_key()
             ctx.undefine(defkey)
             return False
+        return result
     return fn
 
 def check_headers(*headers):