Do not set the global bUseLongDouble flag based purely on sizeof(long double)

being more than 8.  That is a necessary but not sufficient condition to use
the long double routines.  Instead, do a run-time test to verify that long
double really does provide extra precision.

FossilOrigin-Name: aa999d490b743f455f2fb6c41009098330380f0bc2e9b1c28fbc82141a9138ce

diff --cc manifest
index fd46a06a0401871f0e6e283d889b43ff18a8c6f2,806405336effdabebcb2c30d3cc29a04555793c0..5181ab1770ddbc43607e39a8e536f534f371ee4f
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C JNI:\senable\sthe\sfts5-customization\sbits\sby\sdefault\sbut\selide\sthem\sfrom\sthe\sdocs\suntil\sit's\sclearer\swhere\sthey're\sheaded.\sUnrelated\sminor\sdoc\stweaks.
- D 2023-09-13T21:32:20.249
 -C Improved\scomments\son\sthe\shasHighPrecisionDouble()\sroutine.\s\sNo\schanges\sto\nthe\sunderlying\scode.
 -D 2023-09-13T20:35:04.920
++C Do\snot\sset\sthe\sglobal\sbUseLongDouble\sflag\sbased\spurely\son\ssizeof(long\sdouble)\nbeing\smore\sthan\s8.\s\sThat\sis\sa\snecessary\sbut\snot\ssufficient\scondition\sto\suse\nthe\slong\sdouble\sroutines.\s\sInstead,\sdo\sa\srun-time\stest\sto\sverify\sthat\slong\ndouble\sreally\sdoes\sprovide\sextra\sprecision.
++D 2023-09-14T10:46:50.776
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@@ -2120,8 -2120,8 +2120,9 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P 9ea0a9f39d03707d251af5af08d1ba94958704ba74019d7e8823ccd21936023c
- R 15fb56dd0ed96c53b2161c099e5852b9
- U stephan
- Z 08847e36507f621d83fdc14a37bfae39
 -P 9a854b919667e0e679a259542b2ee444ee416dbd73ecd9458f6ced35d9d3f264
 -R 2cf5225da68e808b8d6be96ec87494aa
++P 21adb1aef24398fba918a5e1479ce2b6fd7e86d87eb9f2bea86a0eb43e8809c2 810c635ce063d873e969bf83339c654f6008e84ce8a61f0ffc61806e98d13dde
++R b390e5a75fc36ea63cfccf4d34a2bd01
++T +closed 810c635ce063d873e969bf83339c654f6008e84ce8a61f0ffc61806e98d13dde
+ U drh
 -Z 7ae23f2f1935a56e3223edbdedc613fe
++Z 76fc386c693b15f15cfeb2b7cb92381f
  # Remove this line to create a well-formed Fossil manifest.

diff --cc manifest.uuid
index f4db41a448a440915eee97802bf0e33d7279a427,5b62515d2a0cf7719a8592ddf9abec0ba2718f32..c7ffb9789c1c2160f234df5255008b6ff9cc8474
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 21adb1aef24398fba918a5e1479ce2b6fd7e86d87eb9f2bea86a0eb43e8809c2
 -810c635ce063d873e969bf83339c654f6008e84ce8a61f0ffc61806e98d13dde
++aa999d490b743f455f2fb6c41009098330380f0bc2e9b1c28fbc82141a9138ce