Clean up .appveyor.yml

Whitelist the tagged versions too
Exclude MinGW debug builds since (unlike the VS builds) they use
the same system libraries as the non-debug builds.
We aren't building vs2008 any more anyway.

.appveyor.yml

@@ -6,6 +6,7 @@ branches:
   # whitelist
   only:
     - 3.15
+    - /R3\.15\.\d+.*/
 
 # Version format
 version: base-{branch}-{build}
@@ -35,7 +36,6 @@ clone_depth: 5
 skip_commits:
   files:
     - 'documentation/*'
-    - 'templates/*'
     - '**/*.html'
     - '**/*.md'
     - '.github/*'
@@ -58,6 +58,7 @@ environment:
   # common / default variables for all jobs
   SETUP_PATH: .ci-local:.ci
   BASE: SELF
+  EPICS_TEST_IMPRECISE_TIMING: YES
 
   matrix:
   - CMP: vs2019
@@ -83,14 +84,17 @@ matrix:
     CMP: vs2012
   - platform: x64
     CMP: vs2010
-  - platform: x64
-    CMP: vs2008
   # Exclude more jobs to reduce build time
   # Skip 32-bit for "middle-aged" compilers
   - platform: x86
     CMP: vs2017
   - platform: x86
     CMP: vs2015
+  # MinGW debug builds use the same libraries, unlike VS
+  - configuration: dynamic-debug
+    CMP: gcc
+  - configuration: static-debug
+    CMP: gcc
 
 #---------------------------------#
 #     building & testing          #