Minor static fixes, precheck some variables, build local fix-ca.

Many functions are for internal use, added static to avoid warning.

Reduced the nesting in the switch by prechecking necessay variables
and exiting early.

In Issue 1 it was noted that we can't build fix-ca. Since fix-ca is
just one file, it seemed we can still continue to use gimp-tool-2.0
to build and install the file like before if we avoid including the
fix-ca-config.h - this can be useful for users who need this filter
but do not have admin access.

.github/workflows/main.yml

@@ -64,3 +64,15 @@ jobs:
         run: ./configure
       - name: Make gimp-plugin-fix-ca
         run: make
+  local:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Create configure
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install autoconf automake libtool gcc libgimp2.0-dev
+      - name: Make local gimp-fix-ca install
+        run: gimptool-2.0 --install fix-ca.c
+      - name: Uninstall local gimp-fix-ca
+        run: gimptool-2.0 --uninstall-bin fix-ca