ossfuzz: Move to C++ for curl_fuzzer.

Automake gets confused if you want to use C++ static libraries with C
code - basically we need to involve the clang++ linker. The easiest way
of achieving this is to rename the C code as C++ code. This gets us a
bit further along the path and ought to be compatible with Google's
version of clang.

tests/fuzz/Makefile.am

@@ -30,12 +30,12 @@ AUTOMAKE_OPTIONS = foreign nostdinc
 # $(top_builddir)/lib is for libcurl's generated lib/curl_config.h file
 # $(top_srcdir)/lib for libcurl's lib/curl_setup.h and other "borrowed" files
 
-AM_CFLAGS = -I$(top_srcdir)/include        \
-            -I$(top_builddir)/lib          \
-            -I$(top_srcdir)/lib            \
-            -I$(top_srcdir)/tests/fuzz
+AM_CXXFLAGS = -I$(top_srcdir)/include        \
+              -I$(top_builddir)/lib          \
+              -I$(top_srcdir)/lib            \
+              -I$(top_srcdir)/tests/fuzz
 
-LIBS = -lpthread -lstdc++ -lm
+LIBS = -lpthread -lm
 
 # Run e.g. "make all LIB_FUZZING_ENGINE=/path/to/libFuzzer.a"
 # to link the fuzzer(s) against a real fuzzing engine.
@@ -53,4 +53,4 @@ checksrc:
 	@PERL@ $(top_srcdir)/lib/checksrc.pl $(srcdir)/*.c
 
 noinst_PROGRAMS = $(FUZZPROGS)
-noinst_LIBRARIES = $(FUZZLIBS)
+noinst_LIBRARIES = $(FUZZLIBS)