configure: add workaround for systems without zlib.pc

Some systems (e.g. FreeBSD) might not have installed the appropriate
pkg-config file as they should. We can workaround the issue by creating
the .pc file they should have distributed.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

compat/.gitignore

@@ -0,0 +1 @@
+zlib.pc

compat/gen_zlib_pc.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <zlib.h>
+
+static const char *template =
+	"prefix=/usr\n"
+	"exec_prefix=${prefix}\n"
+	"libdir=${exec_prefix}/lib\n"
+	"\n"
+	"Name: zlib\n"
+	"Description: zlib compression library\n"
+	"Version: %s\n"
+	"Libs: -lz\n";
+
+int main(void)
+{
+	printf(template, ZLIB_VERSION);
+	return 0;
+}

configure

@@ -340,6 +340,15 @@ else
     errors=$((errors + 1))
 fi
 
+if ! pkg-config --exists zlib; then
+  ${CC} ${zlib_cflags} -o compat/gen_zlib_pc \
+	  "$srcdir"/compat/gen_zlib_pc.c ${zlib_ldflags} > /dev/null 2>&1 &&
+  compat/gen_zlib_pc > compat/zlib.pc &&
+  PKG_CONFIG_PATH="$PKG_CONFIG_PATH":compat &&
+  export PKG_CONFIG_PATH
+  rm -f compat/gen_zlib_pc
+fi
+
 printf "Checking for zlib (>= 1.2.5.2)... "
 have_zlib=0
 if pkg-config --atleast-version=1.2.5.2 zlib; then