Add garbage to files for ISO-compliant compilers.

This commit adds a single statement to aprs-is.c and c99math.c that
will allow compilation on ISO-compliant compilers, such as GCC with
the `-pedantic` or `-pedantic-errors` flags. Under a couple certain
conditions -- specifically, when compiling for non-DOS platforms or
with APRS-IS disabled -- two source files would wind up blank after
preprocessing, which is a violation of the ISO C specifications.

This commit adds a single typedef statement to the aforementioned C
source files that does nothing to affect program flow but will make
even the strictest compilers happy.

In the spirit of writing good and portable code (as much as the GNU
extensions will allow), I've also added `-pedantic-errors` into the
configure.ac script, and by extension, into our Makefile. This will
force future maintainers to write high-quality code.

configure.ac

@@ -38,13 +38,14 @@ AC_TYPE_SIZE_T
 AC_TYPE_UINT16_T
 
 dnl  C compiler flags:
-dnl    -std=gnu99    Compile C99, with GNU extensions (for getaddrinfo,
-dnl                  gai_strerror, freeaddrinfo, struct addrinfo)
-dnl    -Wall         Complain heavily.
-dnl    -Wextra       Complain very heavily.
-dnl    -Werror       Make all warnings errors.
-dnl    -O2           Optimize (unless we're debugging)
-AM_CFLAGS="-std=gnu99 -Wall -Wextra -Werror -O2"
+dnl    -O2              Optimize (unless we're debugging)
+dnl    -std=gnu99       Compile C99, with GNU extensions (for getaddrinfo,
+dnl                     gai_strerror, freeaddrinfo, struct addrinfo)
+dnl    -Wall            Complain heavily.
+dnl    -Wextra          Complain very heavily.
+dnl    -Werror          Make all warnings errors.
+dnl    -pedantic-errors Make sure we're writing compliant C code.
+AM_CFLAGS="-O2 -std=gnu99 -Wall -Wextra -Werror -pedantic-errors"
 AM_LDFLAGS="-lm"
 AC_SUBST([AM_CFLAGS])
 AC_SUBST([AM_LDFLAGS])
@@ -194,4 +195,4 @@ OPTIONS:
   APRS-IS support:  $enable_aprs_is
   Debugging output: $enable_debug
 ###########################
-])
+])

src/aprs-is.c

@@ -214,3 +214,9 @@ sendPacket (const char* const restrict server,
 }
 
 #endif /* HAVE_APRSIS_SUPPORT */
+
+/* The following line does nothing. It is present so that ISO-compliant
+   compilers will not complain about this file being empty (which they
+   will if we are not compiling with APRS-IS support. */
+typedef int make_iso_compilers_happy;
+

src/c99math.c

@@ -84,4 +84,10 @@ round (const double x)
 #endif /* round */
 
 #endif /* _DOS */
+
+/* The following line does nothing. It is present so that ISO-compliant
+   compilers will not complain about this file being empty (which they
+   will if _DOS is not defined at compile time. */
+typedef int make_iso_compilers_happy;
+
 #endif /* _APRSWX_C99MATH_H */