Add CFLAGS to Linux release job

Some files in libs are using C compiler, so we need to make sure build
all flags are set for C as well.  Als, add -pipe to avoid temporary
files.

This is intermediary step before merging this step back with build
scripts for other jobs.

.github/workflows/linux.yml

@@ -59,7 +59,7 @@ jobs:
         run: |
           set -x
           ./autogen.sh
-          ./configure CXXFLAGS="-O3 -DNDEBUG"
+          ./configure CFLAGS="-O3 -DNDEBUG -pipe" CXXFLAGS="-O3 -DNDEBUG -pipe"
           make -j "$(nproc)"
           strip src/dosbox
       - name: Package