ardupilotwaf: properly configure double precision tasks

There might be multiple copies of the flags, so remove all of them. Also
pull the setup into a helper function to avoid duplicating the logic.

On ChibiOS boards, this fixes SITL (which had issues with position
quantization) and affects the SBF GPS driver (which was probably fine
before but does change slightly).

Tools/ardupilotwaf/ap_library.py

@@ -277,13 +277,7 @@ def double_precision_check(tasks):
             double_library = t.env.DOUBLE_PRECISION_LIBRARIES.get(src[0],False)
 
             if double_library or src in double_tasks:
-                t.env.CXXFLAGS = t.env.CXXFLAGS[:]
-                for opt in ['-fsingle-precision-constant', '-cl-single-precision-constant']:
-                    try:
-                        t.env.CXXFLAGS.remove(opt)
-                    except ValueError:
-                        pass
-                t.env.CXXFLAGS.append("-DALLOW_DOUBLE_MATH_FUNCTIONS")
+                t.env.CXXFLAGS = ap.set_double_precision_flags(t.env.CXXFLAGS)
 
 
 def gsoap_library_check(bld, tasks):

Tools/ardupilotwaf/ardupilotwaf.py

@@ -146,6 +146,24 @@ def get_legacy_defines(sketch_name, bld):
         'AP_BUILD_TARGET_NAME="' + sketch_name + '"',
     ]
 
+def set_double_precision_flags(flags):
+    # set up flags for double precision files:
+    # * remove all single precision constant flags
+    # * set define to allow double precision math in AP headers
+
+    flags = flags[:] # copy the list to avoid affecting other builds
+
+    # remove GCC and clang single precision constant flags
+    for opt in ('-fsingle-precision-constant', '-cl-single-precision-constant'):
+        while True: # might have multiple copies from different sources
+            try:
+                flags.remove(opt)
+            except ValueError:
+                break
+    flags.append("-DALLOW_DOUBLE_MATH_FUNCTIONS")
+
+    return flags
+
 IGNORED_AP_LIBRARIES = [
     'doc',
     'AP_Scripting', # this gets explicitly included when it is needed and should otherwise never be globbed in