Fix support for “$ CFLAGS=-m32 ./configure.sh” by using CFLAGS for all build invokations of CC.

Without this changeset:

```
cc -Wno-return-type -Wno-implicit-int -I. -L. -o markdown main.o pgm_options.o -lmarkdown 
cc -Wno-return-type -Wno-implicit-int -I. -L. -o mkd2html mkd2html.o -lmarkdown 
cc -Wno-return-type -Wno-implicit-int -I. -L. -o makepage makepage.c pgm_options.o -lmarkdown 
cc -Wno-return-type -Wno-implicit-int -I. -L. -o theme theme.o pgm_options.o -lmarkdown 
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible ./libmarkdown.a when searching for -lmarkdown
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lmarkdown
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible ./libmarkdown.a when searching for -lmarkdown
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lmarkdown
collect2: ld returned 1 exit status
collect2: ld returned 1 exit status
make: *** [markdown] Error 1
make: *** Waiting for unfinished jobs....
make: *** [theme] Error 1
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible ./libmarkdown.a when searching for -lmarkdown
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lmarkdown
collect2: ld returned 1 exit status
make: *** [mkd2html] Error 1
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: skipping incompatible ./libmarkdown.a when searching for -lmarkdown
/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lmarkdown
collect2: ld returned 1 exit status
make: *** [makepage] Error 1
```

with:

```
cc -Wno-return-type -Wno-implicit-int -I. -m32 -L. -o markdown main.o pgm_options.o -lmarkdown 
cc -Wno-return-type -Wno-implicit-int -I. -m32 -L. -o mkd2html mkd2html.o -lmarkdown 
cc -Wno-return-type -Wno-implicit-int -I. -m32 -L. -o makepage makepage.c pgm_options.o -lmarkdown 
cc -Wno-return-type -Wno-implicit-int -I. -m32 -L. -o theme theme.o pgm_options.o -lmarkdown 
```

(no errors).